Zebra GK420 troubleshooting guide: errors, faults and fixes

Short answer: According to Zebra, flashing red indicates that the media has run out or the printhead is open, in other words that the cover is not properly closed; on the GK420t, it also covers the end of the ribbon. Reload the roll or close the cover until it clicks, then press FEED to resume.

Likely cause

The Zebra status light table associates flashing red with the “stopped” state: out of media (labels) or printhead open. On the GK420t, the same indication covers the end of the ribbon (see ribbon out).

Caution: The printhead gets hot during printing: do not touch it and use only the cleaning pen to maintain it. Zebra also points out that electrostatic discharge can damage the printhead and the electronics located under the cover.

What to do

1. Leave the printer switched on: according to Zebra, switching it off while reloading can cause the current data to be lost.
2. Open the printer by pulling the release latches towards you and lift the cover.
3. If the roll is empty, place a new roll on its holders (see loading labels): it must turn freely, without resting on the bottom of the compartment, and the paper must pass under both guides, print side up.
4. Close the cover, pressing down until it snaps into place.
5. Press FEED to resume printing.

Sources: Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Replacing Supplies (opens in a new window) – GK420d Desktop Printer User Guide · Placing the Roll in the Media Compartment (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: The transfer ribbon has run out: the GK420t stops and its light stays red until a new ribbon is loaded. Leave the printer switched on, replace the ribbon, close the cover and press FEED to resume the interrupted job.

Likely cause

Zebra ribbons have a leader (reflector) at the end of the roll that the printer detects to stop printing. The “Replacing Transfer Ribbon” page states that the light turns red and stays red until a new ribbon is loaded; the GK420t status light table classifies ribbon out under flashing red.

Caution: Use only cores with square, undamaged notches: according to Zebra, rounded, frayed or crushed notches can make the ribbon slip, wrinkle it and disrupt ribbon-out detection.

What to do

1. Leave the printer switched on: switching it off during a job causes that job to be lost.
2. Open the top cover and cut the used ribbon so that you can remove the cores.
3. Remove the packaging from the new ribbon and peel off its adhesive strip; place it on the lower supply spindles and turn it until its notches lock onto the left hub.
4. Attach the ribbon leader, using its adhesive strip, to the empty take-up core (reuse the empty core from the previous roll), centring it.
5. Turn the thumbwheel to the left of the supply spindle backwards until the ribbon is taut over the printhead.
6. Close the cover, then press FEED to resume the interrupted job.

Zebra requires certified Zebra ribbons: they carry the end-of-ribbon leader that allows the printer to detect the end of the roll. The ribbon must be at least as wide as the media.

Sources: Replacing Transfer Ribbon (opens in a new window) · Loading Transfer Ribbon (opens in a new window) · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide (official documentation).

Short answer: According to Zebra, solid amber indicates a power-on self test failure, a memory error or a printhead that needs to cool. Just after power-up, amber for about ten seconds is normal; if it lasts, follow the steps below.

Likely cause

Zebra cites three causes: power-on self test (POST) failure, memory error, or a printhead that needs to cool down.

Caution: The printhead gets hot during printing: do not touch it.

What to do

1. At power-up, wait: Zebra states that the light normally stays amber for about ten seconds before turning green.
2. If solid amber appears during printing (possible memory error), switch the printer off and on again and restart the print job.
3. If the printhead needs to cool, switch off the printer for at least five minutes, then switch it back on.

When to call a technician: if solid amber appears just after power-up (self test failure), Zebra asks you to contact an authorised reseller; if it persists after five minutes switched off, the printer needs service.

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide (official documentation).

Short answer: The printhead is overheating. According to Zebra, printing stops until the printhead cools, then resumes automatically: you just need to wait.

Likely cause

Excessive printhead temperature (printhead over temperature), according to the Zebra status light table.

Caution: The printhead gets hot during printing: do not touch it.

What to do

1. Leave the printer switched on and wait for the printhead to cool.
2. Printing resumes by itself; Zebra does not ask for any other action.
3. If the light changes to solid amber, see solid amber light.

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide (official documentation).

Short answer: The printer is paused. Press FEED once to resume printing.

Likely cause

According to Zebra, the printer can be paused following an error or by a programming command. The guide also states that at power-up a flashing green light corresponds to pause mode.

What to do

1. Press FEED once: the printer comes out of pause.
2. Check that the light changes to solid green (printer ready).
3. If the data sent still does not print, see format not recognised.

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Feed Button (opens in a new window) – GK420d Desktop Printer User Guide · Printing a Test (Printer Configuration) Label (opens in a new window) – GK420d Desktop Printer User Guide · The LED on Printer Is Flashing Green but the Printer Is Not Printing – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Status Light Error Resolutions (opens in a new window) (official documentation).

Short answer: Zebra classifies these two states as “needs service”: FLASH memory not programmed (alternating green and red) or critical printhead or motor failure (red, red, green). The guide asks you to return the printer to an authorised reseller.

Likely cause

Alternating green and red: the FLASH memory is not programmed. Red, red, green: critical printhead or motor failure.

What to do

1. Note down exactly the sequence shown by the light.
2. If you switched on the printer while holding down FEED, note that Zebra also describes a slow flash between red and green when the printer is waiting for firmware (see firmware update).
3. In other cases, the guide does not provide for any user action.

When to call a technician: for these two states (FLASH memory not programmed, critical printhead or motor failure), Zebra asks you to return the printer to an authorised reseller.

Sources: Status Light Descriptions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: The printer is defragmenting its memory: this is normal. Do not switch off or reset the printer during the operation; let it finish.

Likely cause

Memory reorganisation (memory defragmentation). Zebra links it to label formats that frequently write to and erase the memory; a reset to factory defaults also ends with a defragmentation.

Caution: According to Zebra, switching off or resetting the printer during defragmentation can damage it.

What to do

1. Leave the printer switched on until the defragmentation has finished.
2. If this state recurs often, review the label formats that frequently write to and erase the memory.

When to call a technician: if the defragmentation repeats endlessly, Zebra asks you to contact technical support.

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Status Light Descriptions (opens in a new window) – GK420d Desktop Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: With the printer switched on, the cover closed and the light solid green, hold down FEED and release it during the sequence of two green flashes. The GK420 detects the media type and length, adjusts its sensors and feeds one to four labels.

Likely cause

Zebra cites media that is not correctly threaded, a printer not calibrated for the loaded media, an unsuitable media sensor or an incorrect top-of-label setting (^LT in ZPL) or length setting (Q in EPL). The guide advises recalibrating as soon as the printer shows unusual symptoms, such as skipped labels.

Official calibration procedure (two flashes)

1. Check that the roll is correctly loaded (see loading labels) and that the cover is closed.
2. With the printer switched on, wait for solid green; if the light is flashing green (pause), press FEED once.
3. Hold down FEED: the green light flashes once, then in sequences of two flashes.
4. Release FEED during the sequence of two flashes: the GK420 starts the standard automatic media calibration.
5. The printer feeds one to four labels, then returns to the ready state.
6. If labels are still skipped, perform a manual calibration.

The guide also states that after power-up, pressing FEED two or three times makes the printer calibrate to the installed media. For programmers, Zebra refers to the selected sensor (^MN) and top of label (^LT) in ZPL, and to the sensor (O) and length (Q) in EPL. The Zebra set-up article for the G Series advises recalibrating every time the label length changes.

Sources: Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · Troubleshooting Tests (opens in a new window) – GK420d Desktop Printer User Guide · Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide · Printing a Test (Printer Configuration) Label (opens in a new window) – GK420d Desktop Printer User Guide · G Series Initial Printer Setup Details and Information – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Printer Setup Overview (opens in a new window) (official documentation).

Short answer: With media loaded and the printer switched on, hold down FEED until the sequence of seven green flashes, then release it. The GK420 adjusts its sensor, prints a sensor profile and saves the new settings.

Likely cause

Zebra recommends manual calibration with pre-printed media, every time the media type, batch or format changes, or when the printer fails its automatic calibration.

Official manual calibration procedure (seven flashes)

1. Load the media.
2. Switch the printer on.
3. Hold down FEED: the green light flashes once, then twice, and so on in sequences; release it during the sequence of seven flashes.
4. The printer adjusts the sensor to the liner used, feeds the media until a label is positioned under the printhead, prints a profile of the sensor settings, then saves these settings.
5. Press FEED: one whole blank label should feed.
6. If it does not, restore the factory defaults (four flashes, see reset) then recalibrate.

According to Zebra, manual calibration disables automatic calibration; resetting to factory defaults (four flashes) restores it.

Sources: Manual Calibration (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Manual Calibration (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: If the GK420 finds no gap, black mark or notch within 1 metre (39 inches) of media, it switches to continuous media mode. Check the loading, clean the sensors, then recalibrate; for difficult media, Zebra advises manual calibration.

Likely cause

Zebra explains that the printer continuously adjusts its detection of label length. If nothing is detected within the default maximum length (1 m), it switches to continuous (receipt) mode until the settings are changed. The guide also provides for cleaning the sensors (gap sensor, black line sensor), on which dust accumulates.

Caution: Do not use an air compressor to clean the sensors: Zebra points out that oil from the pump and water can contaminate the printer and the printhead. A can of compressed air is suitable.

What to do

1. Check the loading: print side up, paper threaded under both guides.
2. Clean the sensors: brush away the dust, if necessary with a dry cotton swab; remove adhesive with a cotton swab moistened with alcohol, then wipe with a dry cotton swab. Repeat until no residue or marks remain.
3. Run a standard calibration again (two flashes).
4. If detection still fails, perform a manual calibration (seven flashes).

For programmers: Zebra advises setting the maximum label length (^ML) to at least twice the longest label; ^MN sets the media type and ^MNA restores automatic detection of all types. On the GK420t, the ribbon-out sensor is cleaned in the same way.

Sources: Media Sensing (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Cleaning the Sensor (opens in a new window) – GK420d Desktop Printer User Guide · Cleaning the Sensor (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Manual Calibration (opens in a new window) – GK420t Desktop Thermal Printer User Guide (official documentation).

Short answer: Open the cover, pull the guides apart and place the roll on its holders, print side up. Bring the paper out at the front, under both guides, then close the cover until it clicks: the GK420 calibrates automatically.

Likely cause

Zebra stresses three points: remove the first length of the roll, which may be dirty or dusty, check that the roll turns freely without resting on the bottom of the compartment, and keep the print side facing up as it passes over the platen roller.

Caution: Electrostatic discharge can damage the printhead and the electronics: Zebra asks you to follow anti-static precautions when working under the cover.

What to do

1. Remove the first length of the roll: it may have become dirty during transport or storage, and this avoids dragging adhesive or dirt between the printhead and the platen roller.
2. Open the printer by pulling the release latches towards you and lift the cover.
3. Pull the media guides apart with one hand, place the roll on the holders and release the guides; the print side must face up as it passes over the platen roller.
4. Pull the paper so that it sticks out at the front of the printer; check that the roll turns freely and does not rest on the bottom of the compartment.
5. Thread the paper under both guides.
6. Close the cover, pressing down until it snaps into place: the printer calibrates automatically.

Sources: Placing the Roll in the Media Compartment (opens in a new window) – GK420d Desktop Printer User Guide · Loading Roll Media (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Thermal Printing (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: Zebra cites a ribbon that is not taut enough, cores with damaged notches, media drifting sideways or a printhead that is not properly latched. After each loading, feed at least 10 cm of media with FEED to remove slack and creases.

Likely cause

The GK420t guide states that rounded, frayed or crushed core notches can make the ribbon slip and cause creases. The Zebra article on print quality for the GX, GK and GC Series adds two causes: media drifting sideways and a printhead that is not latched.

What to do

1. After loading the ribbon and closing the cover, press FEED to feed at least 10 cm (4 inches) of media: slack and creases disappear and the ribbon aligns on its spindles.
2. Check that the ribbon is taut over the printhead: turn the thumbwheel to the left of the supply spindle backwards.
3. Use only cores with square notches in good condition, never the old cores with notches on one side only.
4. Adjust the media guides so that the media does not drift.
5. Check that the printhead is latched on both sides.

The ribbon must be at least as wide as the media: according to Zebra, areas of the printhead not protected by the ribbon wear prematurely.

Sources: Loading Transfer Ribbon (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Resolving Print Quality Issues on GX, GK and GC Series Desktop Printers – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Print Quality Problems (opens in a new window) (official documentation).

Short answer: According to Zebra, labels that do not lie flat on their backing can catch on the guides and rollers, peel off and jam. Clean the platen roller with alcohol; if the jams continue after cleaning, it must be replaced.

Likely cause

Zebra cites poor-quality labels (lifted edges that catch on the guides and rollers) and the build-up of adhesive and residue on the platen roller, the printhead and the paper path, which can lead to lost labels and jams.

Caution: According to Zebra, too much alcohol can contaminate the electronic components and greatly lengthen the drying time.

Cleaning and replacing the platen roller (Zebra procedure)

1. Use good-quality, approved labels that lie flat on their backing.
2. Open the cover (and the peeler door if the printer has one), then remove the media from the platen roller area.
3. Pull the right and left tabs outwards, pivot them upwards, then lift the platen roller out of the chassis.
4. Clean it from the centre towards the ends with a lint-free swab or a lint-free cloth very lightly moistened with medical alcohol of at least 90%; repeat with new swabs if there is a lot of adhesive, and throw them away after use.
5. Refit the roller: bearings in place on the shaft, gear on the left; lower it into the chassis and fold the tabs back down until they click.
6. Wait one minute before closing the peeler door or the cover and reloading the labels.
7. If labels still stick or jam after cleaning, replace the platen roller.

For a long period out of use, Zebra advises leaving a label or a sheet between the printhead and the platen roller, to prevent the printhead from sticking to the roller, and never shipping the printer with a roll installed.

Sources: Replacing Supplies (opens in a new window) – GK420d Desktop Printer User Guide · Platen Cleaning and Replacement (opens in a new window) – GK420d Desktop Printer User Guide · Cleaning (opens in a new window) – GK420d Desktop Printer User Guide · Cleaning and Replacement for the GX, GK and GC Series Platen Roller – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Platen Cleaning and Replacement (opens in a new window) · Long Term Printer Inactivity or Storage (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: Zebra first cites a dirty printhead, then an unsuitable darkness or speed setting, incompatible media, a worn printhead or a slipping platen roller. Clean the printhead with a new cleaning pen, then adjust the darkness with the six-flash sequence.

Likely cause

According to the “Print Quality Problems” table, an incorrect image comes from a printhead that is dirty or too cold, an incorrectly set darkness or speed, incompatible media, a worn printhead, or a platen roller that is losing its grip (foreign matter, glazed surface, cuts). The Zebra article on print quality adds that print that is lighter on one side comes from a printhead not latched on both sides or from a worn roller.

Caution: The printhead gets hot during printing: do not touch it and use only the cleaning pen. A pen that has already been used can carry contaminants that damage the printhead.

Printhead cleaning and settings (Zebra procedure)

1. Let the printhead cool for about one minute.
2. With a new cleaning pen, rub the dark strip of the printhead from the centre towards the ends, to push the adhesive out of the paper path.
3. Wait one minute before closing the printer.
4. With the printer ready, hold down FEED until the sequence of six flashes, then release it: the GK420 prints barcode patterns that get progressively darker. Press FEED once when the pattern is sharp and readable.
5. If the Windows driver or the label software imposes its own darkness or speed, correct these too: they can override the settings made on the printer.
6. If one side is lighter, check that the printhead is latched on both sides.
7. Use media recommended and approved by Zebra; if printing is still poor, clean or replace the platen roller (see platen roller), then consider replacing the printhead.

Zebra recommends cleaning the printhead with each roll of media. By command: darkness ~SD (ZPL) or D (EPL), speed ^PR (ZPL) or S (EPL). The GX, GK and GC Series have no printhead pressure adjustment; according to Zebra, forcing the darkness to compensate for poor media shortens the life of the printhead.

Sources: Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide · Cleaning the Printhead (opens in a new window) – GK420d Desktop Printer User Guide · Cleaning (opens in a new window) – GK420d Desktop Printer User Guide · Adjusting the Print Quality (opens in a new window) – GK420d Desktop Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · Resolving Print Quality Issues on GX, GK and GC Series Desktop Printers – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Print Quality Problems (opens in a new window) (official documentation).

Short answer: On the GK420d, the media is probably not direct thermal: do the scratch test. On the GK420t with ribbon, check that the ribbon is coated on its outer side and that thermal transfer mode is enabled.

Likely cause

Zebra cites media that is not direct thermal when printing without ribbon, incorrectly loaded media and, for printing with ribbon, a ribbon that is not coated on the outside or unapproved media. The GK420d prints only in direct thermal mode: thermal transfer media, which requires a ribbon, is not supported.

What to do

1. Media scratch test: scratch the print side quickly and firmly with your fingernail or a pen cap. A black mark indicates direct thermal media; no mark, thermal transfer media that requires a ribbon.
2. GK420d: use only direct thermal media.
3. GK420t, adhesive test: press the sticky corner of a label onto the outer side of the ribbon roll, then peel it off. If ink remains on the label, the ribbon is coated on the outside and is suitable; if not, it is coated on the inside and cannot be used.
4. With no label to hand, ribbon scratch test: place a piece of ribbon, outer side against a sheet of paper, and scratch its inner side with your fingernail; a mark on the sheet indicates a ribbon coated on the outside.
5. GK420t: check that the print mode is thermal transfer (driver, Zebra Setup Utilities or command); the configuration label must show THERMAL-TRANS under PRINT METHOD.
6. Finally, check the media loading (see loading labels).

Zebra reminds you never to use direct thermal media with a ribbon: barcodes and graphics may be distorted.

Sources: Print Quality Problems (opens in a new window) – GK420T Desktop Thermal Printer User Guide (PDF, P1131978-01EN Rev A) · Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Performing a Media Scratch Test (opens in a new window) – GK420d Desktop Printer User Guide · Ribbon Overview and Use (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Ribbon Test with Adhesive (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Loading Transfer Ribbon (opens in a new window) – GK420t Desktop Thermal Printer User Guide · G Series Initial Printer Setup Details and Information – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Printer Setup Overview (opens in a new window) (official documentation).

Short answer: Zebra cites a dirty printhead or damaged printhead elements. Clean the printhead; if the lines persist, the elements are faulty and the printer needs service.

Likely cause

Dirty printhead or damaged printhead elements, according to the “Print Quality Problems” table. The Zebra article on print quality for the GX, GK and GC Series gives the same causes.

Caution: The printhead gets hot during printing: do not touch it.

What to do

1. Clean the printhead with a new cleaning pen, from the centre towards the ends (see printhead cleaning).
2. Print a configuration label (FEED held down until one flash): the Zebra article advises checking that the box around the text is a continuous line, without any white band.
3. If the white bands persist after cleaning, some printhead elements are faulty.

When to call a technician: if printhead elements are faulty, Zebra states that the printer needs service; the guide describes how to replace the printhead.

Sources: Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide · Print Quality Problems (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Resolving Print Quality Issues on GX, GK and GC Series Desktop Printers – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Print Quality Problems (opens in a new window) (official documentation).

Short answer: Zebra asks you to set the print width when first using the printer and every time the media width changes. Hold down FEED until the sequence of five flashes, then press FEED once when the printed box reaches the required width.

Likely cause

Print width not set for the installed media. For an image cut off on the right, the Zebra article on print quality also cites a ribbon narrower than the label or debris stuck to the printhead.

What to do

1. With the printer ready (light solid green) and the cover closed, hold down FEED until the sequence of five flashes, then release it.
2. The printer prints increasingly wide boxes, in 4 mm steps: press FEED once when the maximum width you want has been reached.
3. Check that the Windows driver or the software (ZebraDesigner, for example) does not impose a different width: they can override this setting.
4. GK420t: use a ribbon wider than the label, and remove any debris or label stuck to the printhead.

By programming: ^PW in ZPL, q in EPL (page mode).

Sources: Adjusting the Print Width (opens in a new window) – GK420d Desktop Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · Resolving Print Quality Issues on GX, GK and GC Series Desktop Printers – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Print Quality Problems (opens in a new window) (official documentation).

Short answer: According to Zebra, too high a darkness widens the bars and narrows the spaces. Lower the darkness, check the quiet zone around the code and make sure that no printhead elements are missing.

Likely cause

The Zebra article on print quality for the GX, GK and GC Series cites three causes: darkness too high, missing printhead elements and insufficient quiet zone. The GK420 guide specifies that beyond the sharp and readable setting, increasing the darkness can distort the bar width.

What to do

1. Lower the darkness: six-flash sequence, and press FEED as soon as the printed pattern is sharp and readable.
2. Allow a quiet zone of at least ten times the width of the narrowest bar.
3. Print a configuration label and check that the box around the text is a continuous line, without any white band; if not, see white lines.

Sources: Resolving Print Quality Issues on GX, GK and GC Series Desktop Printers – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Print Quality Problems (opens in a new window) · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: The printhead is a consumable that wears through contact with the media. With the printer switched off and unplugged, at a workstation protected against static electricity, release the printhead, disconnect its cables, install the new one, clean it, then print a configuration label.

Likely cause

Zebra points out that the printhead wears through friction with the media and that unapproved media can shorten its life or damage it.

Caution: Zebra asks you to switch off and unplug the printer, to protect against electrostatic discharge (grounded conductive mat, anti-static wrist strap) and not to touch the printhead, which gets hot during printing.

Zebra printhead replacement procedure

1. Switch off the printer and unplug the power cord; place the printer on a grounded conductive mat and wear an anti-static wrist strap.
2. Open the printer: release latches pulled towards you, cover raised.
3. GK420d: push the right-hand side latch of the printhead to the right, pull the printhead forwards, then disconnect the two cable harnesses.
4. GK420t: remove the ribbon and the ribbon guide, press the printhead release tabs with a round-ended tool (2.5 to 3.8 mm) through the opening provided, then disconnect the two harnesses and the green ground wire.
5. Connect the new printhead (GK420t: keyed left-hand connector, green ground wire, then right-hand connector) and fit it back in place until it clicks; on the GK420t, refit the ribbon guide and check that the printhead moves up and down freely.
6. Clean the new printhead with a new cleaning pen, from the centre outwards.
7. Reload the media, plug the printer back in, switch it on and print a configuration label to check that it works.

The procedure differs between the GK420d (side latch) and the GK420t (ribbon frame and ground wire); read it in full before you start.

Sources: Replacing the Printhead (opens in a new window) – GK420d Desktop Printer User Guide · Removing and Replacing the Printhead (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: Flashing green means that the printer is receiving data; the light returns to solid green and printing resumes once reception is complete. If nothing comes out, Zebra advises clearing the Windows print queue, restarting the printer, then checking the driver.

Likely cause

The Zebra status light table associates flashing green with receiving data. When the printer receives data without printing, the Zebra article identifies the print queue, the printer's buffer memory, the firmware and the driver as the points to check, testing printing after each step.

What to do

1. Wait for reception to finish: the light should change to solid green.
2. Clear the computer's print queue.
3. Switch off the printer, wait a few seconds and switch it back on.
4. Cancel the contents of the printer's buffer memory, then reset it if necessary (see reset).
5. Update the firmware (see firmware).
6. Reinstall the driver, check its settings and print a test page from the driver; if this test succeeds, look for a conflict with the printing application.

When to call a technician: if the problem persists after all these steps, the Zebra article refers you to Zebra technical support.

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · The LED on Printer Is Flashing Green but the Printer Is Not Printing – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Status Light Error Resolutions (opens in a new window) (official documentation).

Short answer: Zebra cites a paused printer, an incorrectly connected cable, the wrong communication port or, in ZPL, format or command prefixes that do not match. Press FEED if the printer is paused, then check the cable, the port and the prefixes on the configuration label.

Likely cause

According to the “Print Quality Problems” table: printer paused, status light lit or flashing (error), data cable incorrectly installed, wrong port selected; in ZPL, format or command prefix different from that of the format; in EPL, peeler enabled and waiting for the label to be removed, or liner not threaded through the peeler.

What to do

1. If the printer is paused, press FEED.
2. Check the light state in the lookup table.
3. Check that the data cable is correctly installed and that the correct communication port (or the correct USB port) is selected on the computer.
4. In ZPL: print the configuration label (FEED held down until one flash) and check the format and command prefixes; by default, ^ and ~.
5. In EPL with a peeler: remove the waiting label and check that the liner passes correctly through the peeler (see peeler).

Sources: Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide · Print Quality Problems (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Troubleshooting Tests (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: The printer was connected and switched on before the driver was installed. Install the Zebra driver, then update the device driver from Devices and Printers; Zebra recommends installing the driver before connecting the USB cable.

Likely cause

According to Zebra, if the printer is switched on and connected to the computer before the drivers are installed, Windows classifies it as an unspecified device.

Caution: Zebra asks you to keep the printer switched off while connecting the interface cables.

What to do

1. Download and install the Zebra driver for Windows (Zebra Setup Utilities or ZDesigner driver).
2. Open Control Panel, then Devices and Printers.
3. Right-click the incorrectly installed printer, choose Properties, then the Hardware tab.
4. Select the printer in the list of device functions, click Properties, then Change settings and the Driver tab.
5. Click Update Driver, choose to browse the computer for the driver, select the Downloads folder, confirm with OK, then Next.
6. If the printer remains under “Unspecified”, the Zebra article advises uninstalling the corresponding “USB Printing Support” entry in Device Manager, then scanning for hardware changes.

Sources: What to Do If You Forget to Install Printer Drivers First (opens in a new window) – GK420d Desktop Printer User Guide · Installing ZDesigner Driver for USB Printer – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › What to Do If You Forget to Install Printer Drivers First (opens in a new window) · G Series Initial Printer Setup Details and Information – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Printer Setup Overview (opens in a new window) · Installing Drivers and Connecting to a Windows-Based Computer (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) (official documentation).

Short answer: If the label starts with “Now in DUMP”, the GK420 is in communications diagnostics mode: switch it off and on again, or press FEED until “Out of DUMP” is printed. If it prints ZPL or EPL code in plain text, the application is sending printer code through a driver that processes it again.

Likely cause

Zebra describes DUMP mode as a diagnostic tool: the printer prints everything it receives instead of interpreting it. You enter it with the ZPL command ~JD, the EPL dump command, or by switching on the printer with the FEED button held down and releasing it as soon as the light turns green. Second case, according to the Zebra knowledge base: an application that produces ZPL or EPL must not go through the ZDesigner driver, otherwise the code prints as text.

What to do

1. To exit DUMP mode, switch the printer off and on again.
2. Or press FEED several times until the buffer empties and the printer prints “Out of DUMP”.
3. At the next power-up, do not hold down FEED.
4. If the printer prints lines of code without “Now in DUMP”, select the Windows Generic / Text Only driver for the print queue instead of the ZDesigner driver, or send the file directly to the printer.

In the DUMP printout, Zebra flags serial communication errors on blank lines: F (framing), P (parity), N (noise), O (data overrun).

Sources: Communications Diagnostics (opens in a new window) – GK420d Desktop Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · Zebra Printer Prints Command Language Code, Unrecognized Characters or Text – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Communications Diagnostics (opens in a new window) · G Series Initial Printer Setup Details and Information – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Printer Setup Overview (opens in a new window) (official documentation).

Short answer: The IP address appears in the lower half of the configuration label (FEED held down until one flash). For network details, send the ZPL command ~WL: the printer prints a network configuration label.

Likely cause

Zebra states that the IP address is needed to identify and configure the printer on the network. The ZebraNet 10/100 print server is a factory-installed option that provides a browser-based configuration interface.

Caution: Connect the network cable with the printer switched off, as Zebra requires for all interface cables, using a Category 5 or higher UTP RJ45 cable.

What to do

1. Print the configuration label: with the printer ready, hold down FEED until one flash, then release it. The IP address appears in the lower half.
2. For the full network configuration label, send the ZPL command ~WL.
3. Check the Ethernet connector lights: both off, no link; green, 100 Mbit/s link; amber, 10 Mbit/s link; flickering in the other colour indicates activity.
4. Enter the IP address in a browser to open the print server's web pages.
5. To set a fixed address, the Zebra article describes, in these pages: Network Configuration, then TCP/IP Settings, IP protocol set to permanent, saving with Save Current Configuration, then Reset Network and restarting the printer. The login credentials are given in the Zebra print server documentation.

Zebra refers to the ZebraNet 10/100 internal print server manual for advanced network configuration.

Sources: Printer Network Configuration Status Label (opens in a new window) – GK420d Desktop Printer User Guide · ZebraNet 10/100 Internal (Wired) Print Server Option (opens in a new window) – GK420d Desktop Printer User Guide · Interface Cable Requirements (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Using the ZebraNet Print Server Web Page to Configure a Permanent IP Address – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › ZebraNet 10/100 Internal (Wired) Print Server Option (opens in a new window) · Printing a Test (Printer Configuration) Label (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: Zebra points out that Windows sets flow control to “None” by default, whereas the printer requires hardware flow control. Use a null modem (crossover) cable, match the computer's parameters to those of the printer, or reset the serial port to its default values (three flashes).

Likely cause

Different serial parameters between the computer and the printer, in particular flow control. Factory values given by Zebra: 9600 baud, 8 data bits, no parity, 1 stop bit, DTR/XON/XOFF. Current GK420 printers use a null modem cable; the first EPL models used a straight-through cable.

Caution: Zebra asks you to leave the power switch off while connecting the interface cable.

What to do

1. With the printer switched off, connect the cable (male DB-9 connector on the printer side) and tighten its screws.
2. In Windows, set the flow control of the COM port to Hardware.
3. To reset the serial parameters to their defaults: with the printer switched on, hold down FEED until the sequence of three flashes, release it, then press FEED while the light is flashing rapidly amber and green.
4. For automatic synchronisation (autobaud), send ^XA^XZ while the light is flashing amber and green: it changes to solid green once synchronised; no label is printed during this phase.
5. The parameters can also be set with the ZPL command ^SC or the EPL command Y.

G Series printers do not support Windows Plug and Play detection on the serial port.

Sources: Serial Port and Windows Operating Systems (opens in a new window) – GK420d Desktop Printer User Guide · Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · Serial Communications (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) (official documentation).

Short answer: The label taken sensor is not enabled: without it, according to Zebra, the GK420 stacks and ejects the peeled labels. Enable it in the driver or by command, and check that the liner passes between the peeler door and the printer body.

Likely cause

According to Zebra, removing the presented label is what triggers printing of the next one; this operation requires the label taken sensor to be enabled.

What to do

1. Load the labels, close the cover and press FEED until at least 100 mm (4 inches) of labels come out; remove these labels from their liner.
2. Pass the liner over the top of the printer, open the peeler door and slide the liner between the door and the printer body.
3. Close the peeler door and press FEED to advance the media.
4. Enable the label taken sensor: in the driver (with gap media and liner), with ^XA ^MMP ^XZ or ^XA ^JUS ^XZ in ZPL, or with the EPL command OP.
5. During printing, remove each presented label: the printer then prints the next one.

In EPL, Zebra also cites an unrecognised format when the peeler is waiting for a label to be removed or the liner is not threaded through the peeler. The GK420 guide does not mention a cutter option.

Sources: Label Dispenser Option (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Label Dispenser Option (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Print Quality Problems (opens in a new window) – GK420d Desktop Printer User Guide · GK Series Thermal Printers (opens in a new window) – GK420d Desktop Printer User Guide (official documentation).

Short answer: The printer is not receiving power. Check that the power switch is in the on position and check the cables between the socket, the power supply and the printer; then unplug the cord for 30 seconds before plugging it back in.

Likely cause

No electrical power, according to the Zebra status light table.

Caution: Never operate the printer or its power supply in a place where they could get wet: Zebra warns of a risk of serious injury. To cut off the power completely, unplug the cord.

What to do

1. Check that the power switch is pushed up (on).
2. Check the mains cord between the socket and the power supply, then the power supply connector on the printer.
3. Check that the light on the power supply comes on: it indicates that the socket is supplying power.
4. Unplug the cord for 30 seconds, plug it back in, then switch the printer back on.
5. Check that the power supply is the one intended for the GK Series (see power supply).

Sources: Status Light Error Resolutions (opens in a new window) – GK420d Desktop Printer User Guide · Status Light Descriptions and Error Resolutions (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Attaching Power (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) (official documentation).

Short answer: According to Zebra, an unsuitable power supply does not deliver enough power and causes grinding noises and faded print. Use the power supply approved for the GK Series: 100-240 V AC input, 24 V DC and 70 W output.

Likely cause

Insufficient power delivered by a power supply that is not the one intended for the model.

Caution: Zebra requires a three-pin mains cord with an IEC 60320-C13 connector, bearing the certification mark of the country of use, and asks you never to use the printer or its power supply in a damp location.

What to do

1. Check the part number of the power supply connected to the printer.
2. If necessary, replace it with the power supply approved for the GK Series (Zebra part number 105934-053, auto-sensing, PFC compliant, 24 V DC, 70 W).
3. Reconnect the printer and test printing.

Sources: Using the Wrong GX, GK and GC Series Power Supply Causes Light Print or Grinding Noises – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · Attaching Power (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) (official documentation).

Short answer: With the printer switched on and the cover closed, hold down FEED until the sequence of four flashes, then release it. The light turns amber for three seconds, then the GK420 restores its factory settings, calibrates and defragments its memory.

Likely cause

Zebra states that returning to factory defaults can resolve some problems; it also restores automatic calibration disabled by a manual calibration.

Official procedure (four flashes)

1. With the printer switched on and the cover closed, hold down FEED: the green light flashes in sequences.
2. Release it during the sequence of four flashes.
3. The light turns amber for three seconds; if you take no action, the reset takes place automatically.
4. On a printer fitted with the network option, hold down FEED during this phase to choose: releasing after one flash resets only the network, after two flashes only the printer, after three flashes both the printer and the network.
5. The printer then performs a standard calibration followed by a memory defragmentation (light red, amber, green): do not switch it off during this operation.

Command equivalents cited by the GK420t guide: ^JUN for the printer settings, ^JUF for the network options.

Sources: Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · FEED Button Modes (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Resetting the Factory Default Values (opens in a new window) – GK420d Desktop Printer User Guide (PDF, P1129574-01EN Rev A) · Feed Button Modes (opens in a new window) – GK420T Desktop Thermal Printer User Guide (PDF, P1131978-01EN Rev A) (official documentation).

Short answer: When you switch on the printer with the FEED button held down, the light flashes rapidly red: release FEED to enter firmware download mode. The printer is ready when the light flashes slowly between red and green; then send the file, for example with Zebra Setup Utilities.

Likely cause

“Firmware Download” power-up mode described in the FEED button modes. Zebra states that firmware updates are published on its website.

What to do

1. With the printer switched off, hold down FEED and switch it on.
2. When the light flashes rapidly red (amber-red according to the GK420t guide), release FEED: the printer prepares for the download.
3. Wait until the light flashes slowly between red and green.
4. Send the firmware file: in Zebra Setup Utilities, select the printer, open Printer Tools, Actions tab, choose Send file, then the file.
5. The light flashes during the transfer and changes to solid green at the end; print a configuration label to check the update.

The Zebra page on firmware downloads also describes sending via the parallel or serial port (DOS) and via FTP over Ethernet. The Zebra set-up article for the G Series specifies that Zebra no longer provides service or support for these printers.

Sources: Feed Button Modes (opens in a new window) – GK420d Desktop Printer User Guide · FEED Button Modes (opens in a new window) – GK420t Desktop Thermal Printer User Guide · Downloading Printer Firmware to the Desktop & Tabletop Printers – Zebra technical documentation (G Series) (link removed on 21 October 2026: the page is no longer available at its former address) · G Series Initial Printer Setup Details and Information – Zebra knowledge base (support) (link removed on 21 October 2026: the page is no longer available at its former address) · GK420d Desktop Printer User Guide (PDF) › Printer Setup Overview (opens in a new window) (official documentation).