Zebra ZD421 troubleshooting guide: errors, faults and fixes

Short answer: According to Zebra, the cover, which carries the printhead, is not fully closed, or the head-open switch needs servicing. Press both front corners of the cover until the latches click; if the alert persists, call in a technician.

Likely cause

The Zebra guide describes this alert when the printer receives a print command or a FEED button press but cannot continue. Two causes are given: the cover (or the head) is open or not properly closed, or the head-open switch needs repairing.

What to do

1. Press down on both front corners of the top of the printer cover.
2. You should hear and feel the latches click into place: the cover is then locked for printing.
3. If the alert remains displayed even though the cover is properly locked, do not persist: the head-open switch is at fault.

When to call a technician: if the alert persists with the cover latched, Zebra states that the head-open switch must be repaired by a technician.

Sources: Alert: Printhead Open (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printer does not detect any media in the print path: empty roll, label missing on the roll, or printer set for labels while continuous media is loaded. Reload or reposition the media, then press FEED or PAUSE once to resume.

Likely cause

Zebra gives four causes: no roll in the printer; a missing label detected on the roll (a marker the manufacturer places between two labels to signal the end of the roll); a printer set for non-continuous media (labels or black mark) while continuous media is loaded; and finally detection made impossible by corrupted data in memory or a faulty component.

Caution: When a missing label is detected on the roll, Zebra asks you not to switch the printer off: the print job would be lost.

What to do

1. Out of media: load the roll, close the printer, then press FEED or PAUSE once to resume printing.
2. End of roll: remove the remaining media and the core, then install a new roll. Same media: press FEED once. Different media (size, length, supplier or batch): run a SmartCal calibration.
3. Label missing in the middle of the roll: pull the media past the missing label until the next label is over the platen roller, close the printer and press FEED once: the printer resynchronises the label position.
4. Continuous media loaded while the printer expects labels or a black mark: check that the movable sensor is in the default centre position, then calibrate the printer for this media.
5. If the media is still not detected, reload the firmware (see firmware update).

When to call a technician: if the media is still not detected after reloading the firmware, Zebra recommends contacting a technician.

The Zebra pages differ on the STATUS light: solid red according to “Standard Interface Controls”, flashing red according to “Detecting and Recovering from a Media-Out Condition”. In both cases, the SUPPLIES light is red.

Sources: Alert: Media Out (opens in a new window) · Detecting and Recovering from a Media-Out Condition (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printer has detected the end of the ribbon: genuine Zebra ribbons end with a reflective leader that the printer recognises. Replace the ribbon roll (or the cartridge on a ZD421 cartridge model), close the printer and press FEED once.

Likely cause

Zebra first cites the end of the ribbon, on all models: printing stops mid-job, or the alert appears as soon as a job is sent. On ZD421 ribbon cartridge models only, there are two other causes: no cartridge installed (a cartridge is mandatory when the printer is set to thermal transfer), or detection made impossible by dirty contacts on the cartridge chip or on the cartridge sensor, corrupted data in memory or faulty components.

What to do

1. Open the printer: at the end of the ribbon, the reflective leader is visible under the roll or the cartridge, and the take-up roll is full.
2. Remove the used ribbon from the take-up spindles and discard it; keep the empty core to load the next roll.
3. Install a new ribbon roll (see loading the ribbon) or, on a ZD421 cartridge model, a new cartridge (see installing the cartridge).
4. Close the printer and press FEED once; if printing does not resume, press PAUSE.
5. ZD421 cartridge model: if the alert appears while a cartridge is in place, see the cartridge errors entry.

Sources: Alert: Ribbon Out (opens in a new window) · Detecting and Recovering from a Ribbon-Out Condition (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printer is set to direct thermal while a ribbon is installed. If you are printing on direct thermal labels, remove the cartridge without switching the printer off; if you are printing with a ribbon, switch the print method to thermal transfer.

Likely cause

According to Zebra, the alert appears when a job is sent in direct thermal mode while a ribbon cartridge is in place: this mode does not use a ribbon. Second cause: the printer has been left in direct thermal mode while you are printing on thermal transfer media with a ribbon. For ribbon roll models, the guide also states that printing in direct thermal mode with a ribbon loaded causes a “ribbon-in” error.

What to do

1. Direct thermal labels (ZD421 cartridge model): remove the cartridge without switching the printer off, then close it. If printing does not resume, press FEED or PAUSE once.
2. Thermal transfer labels with ribbon: switch the print method to thermal transfer (THERMAL TRANS) from the driver, the labelling software or the commands (see thermal transfer mode).
3. On an Ethernet or Wi-Fi printer, the setting is also available in the print server web pages: Print > Print Quality > Print Type.
4. Check that the label format sent does not contain the ^MTD command (direct thermal) instead of ^MTT (thermal transfer).

The printer has separate heat settings for direct thermal and thermal transfer, designed to give comparable darkness at the same setting level.

Sources: Alert: Ribbon In – ZD421 Ribbon Cartridge Printer Only (opens in a new window) · Standard Interface Controls (opens in a new window) · Loading Zebra Transfer Roll Ribbon (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: On a ZD421 ribbon cartridge model, this signal indicates that the cartridge is nearly empty: the printer estimates that about 10% of the ribbon remains. Have a replacement cartridge ready.

Likely cause

Alert specific to ZD421 ribbon cartridge models: the printer has calculated that about 10% of the ribbon remains in the cartridge, the default alert threshold.

What to do

1. Check that you have replacement cartridges.
2. The alert threshold (10% by default) can be changed with programming commands: Zebra refers you to the “Ribbon Cartridge Programming Commands” section of the guide and to the ZPL programming guide.
3. When the ribbon runs out, replace the cartridge (see installing the cartridge).

Sources: Alert: Ribbon Low – ZD421 Ribbon Cartridge Printer Only (opens in a new window) · Status – Typical Operating Conditions (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Zebra only supports genuine Zebra ribbon cartridges, neither refurbished nor modified. If a genuine cartridge is rejected, the contacts on its chip or the pins of the cartridge sensor are probably dirty: clean them, then try another cartridge.

Likely cause

The “Status – Typical Operating Conditions” page describes the “Cartridge Authentication Failed” state: the cartridge cannot be authenticated or has been modified. When the error appears with an unmodified genuine cartridge, or when “Ribbon Out” is displayed with a cartridge in place, Zebra blames dirty contact pins on the cartridge sensor or dirty chip contacts, or a damaged chip.

What to do

1. Check that it is a genuine Zebra cartridge, neither refurbished nor modified.
2. If the cartridge chip contacts look dirty, clean them with a cotton swab lightly moistened with medical-grade alcohol.
3. If the fault persists, try another cartridge.
4. Also clean the contact pins of the cartridge sensor, following the “Cleaning the Media Path – Upper Half of ZD421 Thermal Transfer Ribbon Cartridge Printers” procedure in the Zebra guide.

Sources: Issue: Erroneous Ribbon Cartridge Faults (opens in a new window) · Status – Typical Operating Conditions (opens in a new window) · Loading the Ribbon Cartridge – ZD421 Ribbon Cartridge Printers Only (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printhead is too hot: the printer pauses to let it cool down, then resumes automatically. If the alert recurs often, move the printer out of direct sunlight or cool the room.

Likely cause

Zebra gives two causes: a large print job (high volume), and an ambient temperature above the printer's operating range, for example when it is exposed to direct sunlight.

Caution: According to Zebra, a hot printhead can cause severe burns: do not touch it until it has cooled down.

What to do

1. Wait: printing resumes by itself once the printhead has cooled down.
2. If the room is too hot or the printer is in full sun, move it to a cooler place or cool the room.

Sources: Alert: Printhead Over Temp (opens in a new window) · Status – Printhead Operation (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printhead is overheating. Zebra asks you to switch the printer off, wait several minutes for it to cool down completely, then switch it back on.

Likely cause

The “Status – Printhead Operation” page describes the “Printhead Shutdown” state: the printhead is over temperature. The “Standard Interface Controls” page associates the STATUS and PAUSE lights flashing red at the same time with printhead overheating, with the same instruction: let it cool down, then restart.

Caution: According to Zebra, a hot printhead can cause severe burns: do not touch it until it has cooled down.

What to do

1. Switch the printer off.
2. Wait several minutes for it to cool down completely.
3. Switch the printer back on.
4. If this happens again, check the causes of overheating that Zebra gives for the Printhead Over Temp alert: large print jobs, room too hot, direct sunlight.

The dedicated alert page (“Alert: Printhead Shutdown”) could not be read: its content was empty when consulted. This entry is based on the pages explaining the lights in the same guide.

Sources: Status – Printhead Operation (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printhead is too cold to print properly, usually because the room is below the minimum operating temperature. Switch the printer off, move it to a warmer place and let it warm up naturally.

Likely cause

Zebra gives two causes: an ambient temperature below the printer's operating range, or a faulty printhead thermistor.

Caution: According to Zebra, a temperature change that is too rapid can cause moisture to condense inside and outside the printer: let it warm up naturally.

What to do

1. Room too cold: switch the printer off, move it to a warmer place and let it warm up naturally.
2. Room at the right temperature: switch the printer off by holding down the POWER button for five seconds, wait until it has shut down completely, then switch it back on.
3. If the error persists, the printhead thermistor is probably faulty.

When to call a technician: if the error persists after a restart in a room at the right temperature, Zebra recommends contacting a technician (faulty printhead thermistor).

Sources: Alert: Printhead Under Temp (opens in a new window) · Standard Interface Controls (opens in a new window) · Status – Printhead Operation (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The printer no longer has enough memory to store the content received (formats, graphics, fonts). Delete unused graphics, fonts or formats, or reduce the label format.

Likely cause

The data cannot be saved to the requested memory location. Zebra distinguishes four types of storage (graphic, format, bitmap and font); the second line of the message indicates the function for which memory is insufficient.

What to do

1. Free up memory by reducing the label format or the printer parameters, to reduce the print area.
2. Delete the graphics, fonts or formats you no longer use.
3. Check that the data is not being sent to a device that is not installed or is unavailable.

Sources: Alert: Out of Memory (opens in a new window) · Standard Interface Controls (opens in a new window) · Status – Typical Operating Conditions (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: According to Zebra, a solid amber STATUS light signals one of four situations: printhead overheating, failure of a printhead element, memory full while saving content, or a power fault on the USB host port or the serial port. The other lights and the context help you decide.

Likely cause

The “Standard Interface Controls” page groups under this light printhead overheating, failure of a printhead element, memory full while saving content and a power fault on the USB host port or the serial port.

How to interpret the light

1. The DATA light is also flashing amber: memory is full, see Out of Memory.
2. Printing stopped during a job: the printhead is overheating, see Printhead Over Temp.
3. The other two causes given by Zebra are failure of a printhead element and a power fault on the USB host port or the serial port.

Source: Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: STATUS shows the general status (solid green: ready; red or amber: error or alert), PAUSE shows pause, DATA shows data exchanges, SUPPLIES shows consumables (media and ribbon) and NETWORK shows the Ethernet or Wi-Fi connection. The lookup table links each state to its entry.

Meaning of the lights according to Zebra

1. STATUS: solid green, ready to print; green slowly turning on and off, sleep mode; solid red, out of media, media detection error, cover or printhead open, cut error or printhead authentication failure; solid amber, printhead overheating, failure of a printhead element, memory full or power fault on the USB host or serial port; flashing amber, printhead too cold; flashing red with PAUSE flashing red, printhead overheating (let it cool down, then restart).
2. PAUSE: solid amber, printer paused; flashing red with STATUS flashing red, printhead overheating.
3. DATA: off, no data transfer; solid green, communication session open with no transfer; flashing green, data transfer in progress; flashing amber, memory full.
4. SUPPLIES: solid red, out of media; flashing red, out of ribbon (thermal transfer); alternating red and amber, ribbon cartridge nearly empty (ZD421 cartridge model); flashing amber, ribbon detected in direct thermal mode (ZD421).
5. NETWORK: solid amber, 10 Mbit/s Ethernet link; solid green, 10/100 Ethernet link or Wi-Fi with strong signal; solid red, Ethernet or Wi-Fi fault; flashing red, Wi-Fi association in progress; flashing amber, Wi-Fi authentication in progress; flashing green, Wi-Fi connected with weak signal.

Sources: Standard Interface Controls (opens in a new window) · Status – Typical Operating Conditions (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Load the media (and the ribbon in thermal transfer mode), close the cover and wait until the STATUS light is solid green. Then press and hold PAUSE and CANCEL together for two seconds: the printer feeds and measures a few labels, then STATUS returns to solid green.

Likely cause

Before printing, the printer must know the media type (labels separated by a gap, black mark or notch, continuous media) and measure its characteristics. Zebra asks for a SmartCal calibration after loading different media (size, length, supplier or batch).

Official SmartCal calibration procedure

1. Load the media and, in thermal transfer mode, the ribbon; close the cover.
2. Press POWER and wait for the ready state: STATUS light solid green.
3. Hold PAUSE and CANCEL down together for two seconds, then release.
4. The printer feeds and measures a few labels, then adjusts its media detection levels.
5. Calibration is complete when the STATUS light returns to solid green.

There is no need to recalibrate for a roll identical to the previous one (same type, supplier, batch and size): the printer measures it and adjusts during printing; press FEED once or twice to synchronise the labels. If the printer does not recognise or does not correctly calibrate the media, Zebra refers you to manual calibration.

Sources: Running a SmartCal Media Calibration (opens in a new window) · Detecting and Recovering from a Media-Out Condition (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Zebra first cites incorrectly loaded media or a badly positioned movable sensor, then sensors not calibrated for the media. Check the loading and the sensor position, run a SmartCal calibration; if the problem persists, clean or replace the platen roller.

Likely cause

Four causes according to Zebra: media loaded incorrectly or movable sensor incorrectly set for the media type; sensors not calibrated for the length, physical properties or detection type of the media (gap/notch, continuous or mark); platen roller slipping or damaged; communication problem (cables or parameters).

What to do

1. Check the media loading and the position of the movable sensor: it must match the media type and the location of the marker (gap, notch or black mark).
2. For a black mark or a notch on the back of the media, the sensor is aligned with the middle of the marker, not with the middle of the roll; for gaps between labels, detection is transmissive, using the central array of the sensor.
3. Run a SmartCal calibration.
4. If labels are still skipped, Zebra suggests a manual media calibration.
5. Clean or replace the platen roller if it slips or is damaged (see platen roller).
6. Check the cables and the communication parameters (see communication diagnostics).

The printer uses transmissive detection (gaps) or reflective detection (marks, notches), never both at the same time.

Sources: Issue: Labels Are Distorted in Size or Print Area Start Position Varies (opens in a new window) · Movable Sensor Adjustment (opens in a new window) · Running a SmartCal Media Calibration (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Zebra recommends cleaning the platen roller as soon as printing or media feed deteriorates: a dirty roller can damage the printhead or cause the media to slip and stick. If the labels still stick or jam after cleaning, the roller must be replaced.

Likely cause

Adhesive, dust, dirt and oils deposited on the roller cause the media to slip or stick and can damage the printhead. The Zebra cleaning schedule states that a slipping roller can distort the printed image or prevent the media from advancing; rollers wear or become damaged over time.

Caution: Linerless roller (labels without liner): do not wash it, rub it or use any cleaning solution, as this would permanently damage its surface. Zebra asks you to clean it only by lightly touching the particles with the adhesive side of a linerless label.

Official roller cleaning procedure

1. Open the cover (and the label dispenser door if there is one), then remove the media from the roller area.
2. Pull the bearing release tabs on the right and left towards the front, then rotate them upwards.
3. Take the roller out of the lower frame, then slide the gear and the two bearings off the shaft.
4. Clean the roller from the centre outwards with a fibre-free swab or a clean, lint-free cloth very lightly moistened with pure 99.7% medical-grade isopropyl alcohol; discard the swab or cloth after use.
5. Repeat with new swabs until the whole surface is clean, especially after an adhesive build-up or a jam.
6. Put the bearings and the gear back on the shaft, then replace the roller in the frame, with the gear on the left.
7. Fold the release tabs back towards the rear, on both sides, until they lock into place.
8. Let it dry for one minute before closing the dispenser door and the cover, and before loading the labels.

The page consulted asks you to replace the roller if the labels still stick or jam after cleaning, without detailing the replacement procedure. Standard rollers: black for 203 dpi, grey for 300 dpi.

Sources: Cleaning and Replacing the Platen (opens in a new window) · Recommended Cleaning Schedule (opens in a new window) · Issue: Labels Are Distorted in Size or Print Area Start Position Varies (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: According to Zebra, a take-up core that does not match the inner diameter of the ribbon roll, or whose notches are damaged, makes the ribbon slip and causes wrinkles. Use an empty core of the same diameter, with intact notches, then feed at least 20 cm of media with FEED to tension the ribbon.

Likely cause

Ribbon cores have an inner diameter of 12.2 mm (0.5 in) or 25.4 mm (1 in); an empty core of a different diameter can cause the ribbon to wrinkle. Damaged notches prevent the core from locking onto the spindle: it slips, causing wrinkles, poor end-of-ribbon detection or intermittent faults. Slack left during loading is the other source of wrinkles that Zebra covers.

Caution: Use a ribbon wider than the media (including the liner) to protect the printhead from wear. According to Zebra, media or ribbons not approved for the printer can damage the printer or the printhead.

What to do

1. Always use an empty core with the same inner diameter as the ribbon roll (12.2 mm or 25.4 mm).
2. Replace cores with damaged notches: square notches are needed to lock the core onto the spindle.
3. Do not fit a 74 m ribbon on the cores of the early desktop printers, which are too large.
4. Attach the ribbon leader straight onto the take-up core, then turn the take-up hub (top towards the back) to remove the slack.
5. With the printer on and the cover closed, press FEED to feed at least 20 cm (8 in) of media: the ribbon tightens, the wrinkles disappear and the ribbon aligns on the spindles.
6. 300 m non-Zebra ribbon: use the Zebra core adapters and a 25.4 mm cardboard core, centre the ribbon and wind at least one and a half turns of ribbon onto the take-up core.

Sources: Thermal Transfer Roll Ribbon Loading (opens in a new window) · Loading Zebra Transfer Roll Ribbon (opens in a new window) · Loading Non-Zebra 300-Meter Transfer Ribbon (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: After loading the ribbon, change the print mode from direct thermal to thermal transfer in the driver, the labelling software or by command (ZPL ^MT). Then print a configuration report: the PRINT METHOD line should show THERMAL-TRANS.

Likely cause

Zebra asks you to change the print mode after loading the ribbon. The printer cannot print in direct thermal mode with a ribbon loaded: attempting to do so causes a “ribbon-in” error.

What to do

1. Check that the ribbon and the media are loaded correctly, then close the cover.
2. Change the print mode from direct thermal to thermal transfer from the printer driver, the application software or the programming commands (ZPL: Media Type command ^MT; EPL in page mode: O command).
3. Print a configuration report (see configuration report): the PRINT METHOD line should show THERMAL-TRANS.

Sources: Loading Zebra Transfer Roll Ribbon (opens in a new window) · Loading Non-Zebra 300-Meter Transfer Ribbon (opens in a new window) · Alert: Ribbon In – ZD421 Ribbon Cartridge Printer Only (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Open the printer, insert the cartridge by the end that carries the chip, holding it by its guide rails, push it in until it clicks, then close the printer. When it is closed, the printer reads the chip: only genuine Zebra ribbon cartridges are supported.

Likely cause

The ZD421 cartridge model prints in thermal transfer mode with a ribbon cartridge. Authentication is done by the chip reader, which reads the cartridge information when the printer is closed.

What to do

1. Open the printer.
2. Hold the cartridge by its guide rails and insert the end that carries the chip into the ribbon transport slot.
3. Push the cartridge in until it is almost flush with the front of the ribbon transport: it locks with an audible and tangible click.
4. Close the printer: it then reads the information stored on the cartridge chip.

Source: Loading the Ribbon Cartridge – ZD421 Ribbon Cartridge Printers Only (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Linerless models print one label at a time: the printer pauses after each label and prints the next one when you remove the previous one. Enable dispensing mode (MEDIA HANDLING set to PEEL-OFF) and never use cleaning solution on the linerless roller.

Likely cause

According to Zebra, the linerless options (tear-off or cutter) work in the same way as on printers with standard media, plus a sensor that detects when the printed label is removed. They require specific cleaning to preserve the platen roller and the non-stick surfaces of the media path.

Caution: Never rub the linerless roller or use any cleaning solution: Zebra states that this damages it permanently. Rings of adhesive on the edges of the roller can come loose and carry particles to the printhead.

What to do

1. Set MEDIA HANDLING to PEEL-OFF in the printer driver, or with the Configure Printer Settings wizard in Zebra Setup Utilities; the setting can also be sent in ZPL (^MMP command).
2. Remove each printed label: the printer then prints and presents the next one, until the end of the job.
3. Only clean the linerless roller if particles are visible, removing them with the adhesive side of a linerless label.
4. Replace the linerless roller if the media sticks or jams: its non-stick coating may be worn.

Linerless rollers: reddish-brown for 203 dpi, brown for 300 dpi. The shared ZD421 / ZD621 guide does not specify which ZD421 part numbers exist in a linerless version.

Sources: Using the Linerless Options (opens in a new window) · Recommended Cleaning Schedule (opens in a new window) · Cleaning and Replacing the Platen (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Remove debris, dust and crust from the holders, guides and surfaces of the media path with a swab or lint-free cloth lightly moistened with pure 99.7% medical-grade isopropyl alcohol. Let the alcohol evaporate completely before reloading.

Likely cause

The Zebra cleaning schedule provides for cleaning the media path as needed, with fibre-free swabs or cloths moistened with 99.7% isopropyl alcohol, letting the alcohol evaporate completely.

Caution: Zebra specifies that the printhead, the movable sensor and the platen roller must not be cleaned as part of this procedure. Too much alcohol can contaminate the electronic components and greatly lengthen the drying time. Do not use an air compressor instead of a can of compressed air: it blows out microparticles that can damage the printer.

What to do

1. Remove debris, dust and crust from the roll holders, the guides and the surfaces of the media path with a swab or lint-free cloth.
2. Lightly moisten the swab or cloth with pure 99.7% medical-grade isopropyl alcohol.
3. For difficult areas, soak a swab more thoroughly to loosen adhesive deposits in the media compartment.
4. Let the alcohol evaporate completely.
5. The printhead, the movable sensor and the platen roller are cleaned separately, following their own procedures (see printhead and platen roller).

Sources: Cleaning the Media Path (opens in a new window) · Recommended Cleaning Schedule (opens in a new window) · Cleaning (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Zebra recommends cleaning the printhead each time a new roll of media is loaded, with a new cleaning pen. Rub the dark area of the printhead from the centre outwards, then wait one minute before closing the printer or reloading the ribbon.

Likely cause

The Zebra guide presents printhead cleaning as necessary for optimal printing. Cleaning from the centre outwards pushes adhesive coming from the edges of the media out of the media path. A pen that has already been used carries contaminants that can damage the printhead.

Caution: The printhead heats up during printing: do not touch it, to avoid damaging it and injuring yourself. Observe anti-static precautions: electrostatic discharge can damage or destroy the printhead and the electronic components under the cover. Use the cleaning pen only for the printhead.

Official printhead cleaning procedure

1. ZD421t (ribbon roll): if a ribbon is installed, remove it.
2. ZD421 cartridge model: lift the printhead actuator arm to access the printhead.
3. Rub a new cleaning pen across the dark area of the printhead, from the centre outwards.
4. Wait one minute for everything to dry before closing the printer or reloading the ribbon.
5. ZD421 cartridge model: release the actuator arm, then push the ribbon transport against it until the release arms lock into place.
6. If the print is still faded after cleaning, adjust the darkness (see darkness setting).

Frequency: the printhead cleaning page recommends cleaning with each new roll, whereas the Zebra cleaning schedule specifies cleaning every five rolls printed. Both instructions appear in the official guide.

Sources: Cleaning the Printhead (opens in a new window) · Cleaning the Printhead – ZD421/ZD621 Thermal Transfer Printers (opens in a new window) · Cleaning the Printhead – ZD421 Ribbon Cartridge Printers (opens in a new window) · Recommended Cleaning Schedule (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Print a print quality report (FEED self test): the printer prints a series of labels at different darkness settings and at two speeds. Keep the setting that gives the best barcodes, then apply it with Zebra Setup Utilities, the labelling software or the manual darkness adjustment.

Likely cause

According to Zebra, print quality depends on the printhead heat setting (darkness), the print speed and the media used: these settings are adjusted together. Each media requires its own darkness setting, and the media manufacturer may recommend a speed lower than the printer's maximum speed.

What to do

1. First print a configuration report (CANCEL self test) to check the media settings.
2. Run the print quality report (FEED self test): the series starts three steps below the current darkness (-3) and rises to three steps above it (+3), once at low speed and once at high speed (51 and 152 mm/s at 203 dpi, 51 and 102 mm/s at 300 dpi).
3. Each label shows its relative darkness and its speed: compare the barcodes and keep the best-rated combination for your media.
4. Apply the setting with the Configure Print Quality wizard in Zebra Setup Utilities; the speed can also be set in the labelling software (for example ZebraDesigner) or with the ZPL command ^PR.
5. Another method is manual darkness adjustment in advanced mode: the printer prints a test pattern for each darkness level; confirm the pattern whose black lines are solid and even to save that value.

Sources: Printing a Print Quality Report (FEED Self Test) (opens in a new window) · Adjusting the Print Quality (opens in a new window) · Manual Print Darkness Adjustment (opens in a new window) · Printing the Printer and Network Configuration Reports (CANCEL Self Test) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: According to Zebra, the ribbon material (wax, wax-resin or resin) may not be suitable for the media (paper, coated paper or synthetic). Use a ribbon suited to the media and do not exceed the maximum print speed recommended for the ribbon.

Likely cause

Incompatibility between the ribbon material and the media material, or a print speed above the maximum speed recommended for the ribbon.

What to do

1. Check that the ribbon matches the media: Zebra offers wax (Performance Wax), wax-resin (Premium Wax/Resin) and resin (Performance Resin for synthetics and coated paper, Premium Resin for synthetics) ribbons, with different maximum speeds.
2. Set the printer so as not to exceed the maximum speed recommended for the ribbon.
3. ZD421 cartridge model: the cartridge characteristics can be read with the commands described in “Ribbon Cartridge Programming Commands”.

Sources: Issue: General Print Quality Issues (opens in a new window) · Thermal Transfer Roll Ribbon Loading (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: In direct thermal mode, the label must have a heat-sensitive coating. According to Zebra, a thermal transfer label printed without a ribbon comes out blank. Scratch the label with your fingernail: if no mark appears, it is not a direct thermal label.

Likely cause

Direct thermal labels have a coating that turns black with heat. Thermal transfer labels do not: printed without a ribbon in direct thermal mode, they remain blank.

What to do

1. Do the fingernail test: scratch the surface of the label; a direct thermal label will mark.
2. If no mark appears, consult your label supplier, or switch to thermal transfer mode with a ribbon (ZD421t or ZD421 cartridge model).
3. In thermal transfer mode, check that the print mode is set correctly (see thermal transfer mode).

Sources: Testing Labels for Direct Thermal Printing Applications – Zebra knowledge base (link removed on 21 October 2026: the page is no longer available at its former address) · ZD421 and ZD621 Series Desktop Printers User Guide (PDF) › Determining Thermal Media Types (opens in a new window) (official documentation).

Short answer: According to Zebra, direct thermal paper is very sensitive to heat and light: if overexposed, it turns black and makes the text or barcode unreadable. It is suitable for short-life labels (shipping, receipts, tickets, patient or visitor identification); for a durable label, Zebra points you to thermal transfer.

Likely cause

Direct thermal paper turns black under the effect of heat, light or other catalysts. Zebra states that it is not suitable for environments exposed to heat, prolonged direct sunlight or abrasion, nor for lifetime identification.

What to do

1. Check the conditions the label will be exposed to: heat, long exposure to direct sunlight or friction degrade direct thermal labels.
2. For exposed labels, Zebra mentions top-coated direct thermal labels that are more resistant to UV, chemicals and abrasion: discuss this with your label supplier.
3. For durable identification (product identification, asset inventory, laboratory, cold rooms, outdoors), switch to thermal transfer with a ribbon suited to the media (ZD421t or ZD421 cartridge model).

Zebra also states that printheads in thermal transfer printers last longer than those in direct thermal printers.

Source: What Is the Difference Between Direct Thermal and Thermal Transfer Printing? (opens in a new window) – Zebra FAQ (Resource Library) (official documentation).

Short answer: Unplug the printer, let it cool down and work at a station protected against static electricity. Release the printhead with its latch, disconnect its connectors, install the new printhead, clean it with a new pen, then print a configuration report.

Likely cause

Zebra provides a procedure for each variant: direct thermal (ZD421d), thermal transfer with ribbon roll (ZD421t) and thermal transfer with ribbon cartridge.

Caution: Zebra asks you to unplug the printer and let it cool down to avoid injury and damage to the circuits, and to work at an anti-static workstation (earthed conductive mat, conductive wrist strap).

What to do

1. Prepare an anti-static workstation: printer placed on an earthed conductive cushioning mat, conductive strap on your wrist.
2. Switch the printer off, unplug it and let it cool down; open the cover.
3. Push the printhead release latch (towards the printhead on the ZD421t, away from the printhead on the ZD421d): the right side of the printhead is released.
4. Free the printhead (ZD421t: to the right and downwards, out of the ribbon carriage; ZD421d: to the right, out of the cover), then firmly but gently disconnect the two cable harnesses and, on the ZD421d, the earth wire.
5. Connect the new printhead: right connector first (keyed, fits one way only), earth wire on the ZD421d, then left connector.
6. Put the left side of the printhead back in place, then push the right side until it is locked by the latch; check that the printhead moves up and down freely under pressure and stays locked when you release it.
7. Clean the new printhead with a new cleaning pen, from the centre outwards, to remove fingerprints and debris.
8. Reload the media, reconnect the power cord, switch the printer on and print a configuration report to check that it is working.

The procedure specific to ZD421 ribbon cartridge models (“Replacing the Printhead – ZD421 Thermal Transfer Ribbon Cartridge Printer Models”) was not read for this guide.

Sources: Replacing the Printhead (opens in a new window) · Replacing the Printhead – ZD421/ZD621 Thermal Transfer Ribbon Roll Printer Models (opens in a new window) · Replacing the Printhead – ZD421/ZD621 Direct Thermal Printer Models (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The installed printhead is not a genuine Zebra printhead, or the printer cannot read its resolution (dpi) because it was replaced incorrectly. Install a genuine Zebra printhead following the replacement procedure.

Likely cause

According to Zebra, “Printhead Resolution Error” means that the printer cannot read the printhead resolution type, which happens if the printhead was replaced incorrectly or replaced with a non-Zebra printhead; “Unauthorized Printhead Error” means that the installed printhead is not a genuine Zebra printhead. The “Standard Interface Controls” page lists printhead authentication failure among the causes of a solid red STATUS light.

What to do

1. Check that the installed printhead is a genuine Zebra printhead; if not, install one to be able to continue.
2. If it is indeed a Zebra printhead, redo the fitting following the replacement procedure.

Sources: Status – Printhead Operation (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The label format was not recognised and no data arrived: according to Zebra, the communication parameters are incorrect. Check the communication settings in the driver or the software and, for a serial connection, the port parameters and the cable.

Likely cause

Incorrect communication parameters between the computer and the printer.

What to do

1. Check the communication parameters in the printer driver or the software, if applicable.
2. Serial connection: check that the flow control (handshake) protocol and the serial port parameters of the printer match those of the computer.
3. Serial connection: check the cable; it may not be a standard DTE or DCE cable, may be damaged or may exceed the maximum length specified by the RS-232 standard.
4. With USB, if the printer was connected before the driver was installed, see USB driver.

Sources: Issue: Label Job Sent, No Data Transfer (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The data is arriving but the format is not recognised. Zebra gives three causes: different prefix and delimiter characters between the printer and the format, computer communication parameters different from those of the printer, or an incorrect label format.

Likely cause

Label format received but not recognised: different prefix and delimiter, different communication parameters, or incorrect format.

What to do

1. Check the printer's ZPL command prefix (COMMAND CHAR) and delimiter (DELIM./CHAR) characters; the correspondence with the commands is given in “Configuration Setting to Command Cross-reference”.
2. Check that the computer's communication settings match those of the printer.
3. Check the label format using the Zebra ZPL programming guide.

Sources: Issue: Label Job Sent, Data Transfers, But No Print (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Run the communication diagnostics test: press and hold PAUSE and FEED together for two seconds. The printer then prints the data received as ASCII text with its hexadecimal values; the codes FE, OE, PE or NE indicate a communication error.

Likely cause

The test checks the link between the printer and the computer or host device: it shows exactly what the printer receives.

Official communication diagnostics test procedure

1. Load the media and switch the printer on.
2. Set the print width to a value equal to or less than the width of the test label.
3. Press and hold PAUSE and FEED together for two seconds: the STATUS light alternates between green and amber, and the printer is in diagnostics mode.
4. The printer prints the data received as ASCII characters, with the hexadecimal values below (including control codes, such as CR); the test label comes out upside down.
5. Look for the error codes: FE (framing error), OE (overrun error), PE (parity error), NE (noise). If there are any, check the communication parameters.
6. To exit diagnostics mode, press and hold PAUSE and FEED again for two seconds, or switch the printer off and on again.

Source: Performing a Communication Diagnostics Test (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: First look at the two lights on the RJ-45 connector: if both are off, no Ethernet link is detected; then check the cable (RJ-45 UTP, category 5 or higher) and the connection. A solid red NETWORK light signals an Ethernet fault. The printer must also be configured to connect to the network, via the web pages of its print server.

Likely cause

According to Zebra, the printer must be physically connected to the network and configured to establish the connection. On the ZD421, the modular slot takes either a 10/100 Ethernet module or an RS-232 serial port, which can be installed on site.

What to do

1. Check that the printer is fitted with the Ethernet module: on the ZD421, 10/100 Ethernet is a module to be installed on site in the modular slot.
2. Use a category 5 or higher RJ-45 UTP Ethernet cable.
3. Read the RJ-45 connector lights: both off, no link; green, 100 Mbit/s link; green with flashing amber, 100 Mbit/s link with activity; amber, 10 Mbit/s link; amber with flashing green, 10 Mbit/s link with activity.
4. Read the NETWORK light: off, no Ethernet link; solid green, 10/100 link; solid amber, 10 Mbit/s link; solid red, Ethernet fault (printer not connected).
5. Configure the connection in the web pages of the built-in print server; for this, Zebra refers you to the “Wired and Wireless Print Server User Guide”.
6. Print a configuration report (see configuration report): the network report prints at the same time.

The Zebra guide does not give a user name or password at this point: for access to the print server web pages, refer to the Zebra print server guide.

Sources: Ethernet (LAN, RJ-45) (opens in a new window) · Status – Ethernet (LAN) Option (opens in a new window) · Standard Interface Controls (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The NETWORK light shows the stage at which the connection is stuck: flashing red during network association, flashing amber during authentication, flashing green if the signal is weak, solid red in the event of an error (printer not connected). Wi-Fi is an option: first check that the wireless module is installed.

Likely cause

According to Zebra, the printer goes through two phases, association then authentication, before it is connected; a link error leaves it off the network. Wi-Fi (802.11ac) and Bluetooth Classic 4.1 are options installed at the factory or on site.

Caution: Before installing the wireless module, Zebra asks you to remove the media, switch the printer off and disconnect all power and interface cables, observing anti-static precautions.

What to do

1. Identify the state of the NETWORK light: flashing red, network association in progress; flashing amber, authentication in progress; solid green, connected with a strong signal; flashing green, connected with a weak signal; solid red, error, printer not connected.
2. If the wireless module was added on site, check that it is pushed fully home: according to Zebra, the module door only closes properly if the module is correctly in place.
3. Print a configuration report (see configuration report): the network report prints at the same time.
4. To change the network settings, Zebra refers you to the print server web pages; the guide states that the print server must be reset for network changes to take effect.

Sources: Status – Wi-Fi (WLAN) Option (opens in a new window) · Standard Interface Controls (opens in a new window) · Installing the Wireless Connectivity Module (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: If the printer is switched on and connected before the drivers are installed, Windows classifies it as an unspecified device. Install the Zebra driver, then assign it to the printer from Devices and Printers: Properties, Hardware tab, update driver.

Likely cause

The Zebra guide specifies installing the drivers before connecting the printer. If it is powered on before, Windows shows it as an unspecified device.

What to do

1. Open the Windows Control Panel, then Devices and Printers.
2. Right-click the incorrectly identified Zebra printer and open its Properties.
3. In the Hardware tab, select the printer in the list of device functions and open its properties.
4. In the driver settings, choose to update the driver by browsing the computer for the software.
5. Point to the Downloads folder and confirm: the printer receives the correct driver and appears under its name.
6. Use a USB cable bearing the “Certified USB” logo, which guarantees USB 2.0 compliance.

Sources: What To Do If You Forget to Install the Printer Drivers First (opens in a new window) · USB Interface (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: The cutter blade is jammed, usually by media, adhesive or a foreign object. Switch the printer off by holding POWER for five seconds, wait until it has shut down completely, then switch it back on; if the error persists, call in a technician.

Likely cause

The cutter blade is stuck and does not move correctly: media, adhesive or a foreign object is blocking it. The “Status – Typical Operating Conditions” page refers to a binding blade (Cut Error, Binding).

Caution: The cutter contains no user-serviceable parts. Never remove its cover and never insert objects or your fingers into the mechanism. According to Zebra, unapproved tools, cotton buds and solvents (including alcohol) can damage the cutter, shorten its life or cause it to jam.

What to do

1. Switch the printer off by holding down the POWER button for five seconds.
2. Wait until it has shut down completely.
3. Switch the printer back on.
4. If the error persists, do not work on the mechanism: contact a technician.

When to call a technician: if “Cutter Error” persists after a restart, Zebra asks you to contact a technician: the cutter cannot be repaired by the user.

Sources: Alert: Cutter Error (opens in a new window) · Status – Typical Operating Conditions (opens in a new window) · Standard Interface Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Only clean the plastic surfaces of the media path, at the cutter's entry and exit, with a swab or lint-free cloth lightly moistened with 99.7% isopropyl alcohol. Never clean the blade or the mechanism: their special coating can be damaged.

Likely cause

According to Zebra, the cutter blade requires no maintenance cleaning: it has a special coating that resists adhesives and wear, and cleaning can damage it.

Caution: The cutter contains no user-serviceable parts. Never remove its cover and never insert objects or your fingers into the mechanism. According to Zebra, unapproved tools, cotton buds and solvents (including alcohol) can damage the cutter, shorten its life or cause it to jam.

What to do

1. Clean the ribs and plastic surfaces of the media entry (inside) and the exit slot (outside) of the cutter, with a clean swab or lint-free cloth lightly moistened with 99.7% isopropyl alcohol.
2. Repeat if necessary to remove adhesive residue, then let it dry.
3. For any other cleaning or maintenance of the cutter, Zebra refers you to a technician; only the outer cover is cleaned like the outside of the printer.

The Zebra cleaning schedule says to clean only the cutter cover, whereas the “Cleaning the Cutter Option” page also describes cleaning the plastic entry and exit surfaces. Both forbid touching the blade.

Sources: Cleaning the Cutter Option (opens in a new window) · Recommended Cleaning Schedule (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: If the label taken sensor is not enabled, peeled labels build up in the dispenser and can jam the mechanism. Enable this sensor with a programming command: the printer then waits for you to remove each label before presenting the next one.

Likely cause

In dispensing mode, the printer peels off the liner and presents the label. Without the label taken sensor enabled, it does not know that the label has been removed and the labels pile up.

Caution: According to Zebra, without the label taken sensor enabled, piled-up labels can jam the printer mechanism.

What to do

1. Set Media Handling to Peel-Off in the Windows driver or with the Configure Printer Settings wizard in Zebra Setup Utilities (in ZPL: ^MMP).
2. Load the labels, close the printer and press FEED until at least 100 mm of labels come out; leave them on their liner.
3. Pass the liner over the top of the printer and open the dispenser door by pulling its gold central latch.
4. Slide the liner between the dispenser door and the printer body, then close the door while pulling the end of the liner taut.
5. Press FEED once or more until a label is presented.
6. Enable the label taken sensor with a programming command, then remove each label so that the next one prints.

Source: Using the Label Dispenser Option (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Solid red health light: internal battery error; flashing red: charging error (temperature too high or too low, internal monitoring error…). Let the battery return to room temperature and check its charge; if the fault persists, it may be at the end of its life or faulty and must be replaced.

Likely cause

Zebra gives three causes: battery too hot or too cold, battery at the end of its useful life, faulty battery component.

What to do

1. In the event of an internal error (solid red), remove the battery from the printer, as Zebra requests.
2. If the battery is too hot or too cold, let it return to room temperature, then check its charge again.
3. Remove it and recharge it to check its state of charge: it may have reached the end of its life.
4. If a battery component is faulty, replace the battery.

Sources: Issue: Battery Displays a Red Indicator (opens in a new window) · Battery Indicators and Controls (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: When no charge bars are lit, the battery needs recharging and the printer cannot be switched on. Batteries are also shipped in shutdown mode: press the battery button to wake it up, then switch the printer on within one minute; a full charge takes about two hours.

Likely cause

According to Zebra, batteries are shipped in shutdown mode, for safety and to prevent them discharging during storage and transport. A battery with no charge bars lit must be recharged: the printer cannot start in this state.

What to do

1. Briefly press the battery control button: it comes out of sleep or shutdown mode and checks its status; all the lights flash three times together.
2. Switch the printer on within the next minute, otherwise the battery returns to its previous mode.
3. Read the charge level, displayed for 10 seconds: three solid green bars, fully charged; two bars with the top one flashing, partially charged; one flashing bar or no bars, recharging needed; amber, charging in progress.
4. Recharge the battery with the printer's power supply or another approved Zebra power source: allow about two hours; the status light changes from amber (charging) to green (charged).
5. To put the battery back into shutdown mode (storage, transport), hold its button down for 10 to 11 seconds: all the lights flash three times to confirm.

Sources: Battery Indicators and Controls (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: This is not a fault: according to Zebra, a green STATUS light that slowly turns on and off indicates that the printer is in sleep mode. The POWER button controls sleep and wake-up; a short press puts the printer into Energy Star low-power mode.

Likely cause

Energy Star sleep mode: after a period of inactivity, the printer goes into sleep mode to reduce its power consumption.

What to do

1. Use the POWER button: it switches the printer on and off and controls sleep and wake-up.
2. To switch the printer off completely, hold down POWER for 4 to 9 seconds.
3. For it to restart by itself when power returns after an outage, Zebra describes a recovery mode (Power Failure Recovery Mode), which requires a connectivity module.

Sources: Standard Interface Controls (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Download the firmware from the printer's Zebra support page, then send it with Zebra Setup Utilities: Open Printer Tools, Action tab, Send file. The DATA light flashes green during the download; the printer restarts and prints a configuration report at the end.

Likely cause

Zebra recommends updating the firmware regularly to benefit from new features, improvements and changes to media handling and communications. The guide also asks for an update after options have been installed.

Official update procedure

1. Download the firmware from your printer's Zebra support page.
2. Open Zebra Setup Utilities and select the printer.
3. Click Open Printer Tools, then open the Action tab.
4. Load media into the printer.
5. Click Send file, then Browse, and select the downloaded firmware file.
6. Wait until it finishes: the DATA light flashes green during the download, then the printer restarts with all the lights flashing.
7. The STATUS light turns solid green during validation and installation; the printer prints a configuration report when the update is complete.

The firmware is only downloaded if its version differs from the one already installed.

Sources: Updating Printer Firmware (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Most often, the settings were changed without being saved (ZPL command ^JU) before switching off, or a command in the label format overwrites them. Check the command syntax and the prefix characters; if the problem persists, restore the factory settings and then reload the firmware.

Likely cause

Zebra gives four causes: settings changed without being saved with ^JU; syntax errors or misuse of format or direct commands (a command can prevent a parameter from being changed or reset it to its default value); different prefix and delimiter characters between the printer and the format; a main logic board problem (corrupted firmware or repair needed).

What to do

1. Turn the printer off and on again to check whether the settings have been saved.
2. Check the use and syntax of the commands in the ZPL programming guide.
3. Check that the printer's Control, Command and Delimiter characters match your software environment, by comparing them with a configuration report.
4. Restore the factory settings (see reset), for example with Zebra Setup Utilities (Open Printer Tools, Action: Load printer defaults).
5. Reload the firmware (see firmware update).

When to call a technician: if the printer does not return to normal operation after restoring factory settings and reloading the firmware, Zebra refers you to a technician: the main logic board is not user-serviceable.

Source: Issue: Settings are Lost or Ignored (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: According to the Zebra guide, hold down FEED and PAUSE together while switching on to restore the printer parameters to their factory values. You can also use Zebra Setup Utilities (Open Printer Tools, Action: Load printer defaults). All manually changed settings will need to be redone.

Likely cause

Zebra distinguishes between restoring the printer settings (all except network), resetting the wired or wireless print server (the printer then re-associates with the wireless network) and reloading the last saved configuration.

Caution: After a return to default values, all manually changed settings must be redone. On a printer in protected mode, this setting cannot be changed from the control panel.

What to do

1. Note the settings you have changed: they will need to be redone.
2. Hold down FEED and PAUSE together while switching the printer on to restore the factory parameters.
3. Alternative method: Zebra Setup Utilities, Open Printer Tools, Action tab, Load printer defaults.
4. By ZPL command: ^JUF restores the printer's factory settings (excluding network), ^JUN resets the wired or wireless print server, ^JUR reloads the last saved configuration.
5. Print a configuration report to check the result (see configuration report).

Sources: System > Settings > Restore Defaults (opens in a new window) · ZD421 and ZD621 Series Desktop Printers User Guide (P1123730-05EN Rev A, PDF) (opens in a new window) · Issue: Settings are Lost or Ignored (opens in a new window) – ZD421 / ZD621 user guide (official documentation).

Short answer: Load the media, close the cover and switch the printer on. When the STATUS light is solid green, press and hold FEED and CANCEL together for two seconds: the printer prints the printer configuration report and the network configuration report.

Likely cause

Zebra recommends printing this report before connecting the printer to a computer, to check that it is working; it also helps with installation and troubleshooting. If the reports do not print, the guide refers you to its troubleshooting chapter.

Official procedure (CANCEL self test)

1. Load the media (and the ribbon in thermal transfer mode), then close the cover.
2. Switch the printer on and wait until the STATUS light is solid green.
3. Press and hold FEED and CANCEL together for two seconds, then release.
4. The printer and network configuration reports print; the STATUS light returns to solid green at the end.

Sources: Printing the Printer and Network Configuration Reports (CANCEL Self Test) (opens in a new window) · Test Printing with a Configuration Report (opens in a new window) – ZD421 / ZD621 user guide (official documentation).