Prusa MK4 Filament Not Feeding: Choose the Next Check

When filament stops moving, start by identifying where it stops: at the spool, while entering the extruder, or after the gears have taken hold. A clicking sound or a broken strand is an observation to record, not a diagnosis by itself.

This guide covers the original Prusa MK4 with its standard Nextruder, without MMU3. Use it to choose the next manufacturer procedure. Modified feed systems, flexible materials and other printer models need their own instructions.

Record the symptom before changing anything

Pause a failing job using the printer controls. Note the material, nozzle type, firmware version and anything recently changed. Record whether the problem occurs during loading or during a print, whether filament reaches the extruder, and whether anything comes out of the nozzle. Photograph an error message or the visible obstruction before clearing it.

What you can observe	Start here
A strand caught under another loop on the spool	Spool check
The printer tries to load, but does not take in the tip	Loading check
Filament is taken in, but output stops or is inconsistent	Nozzle troubleshooting
The screen reports stuck filament	Follow the message, then check for recurrence
Filament is present but the printer does not recognize it	Filament-sensor troubleshooting

This order is an inspection plan, not a claim that one cause is most common.

1. Check the spool before opening the extruder

Look for an actual crossed loop trapping the feeding strand. Prusa's tangled-filament guide explains how a loose filament end can slip under another loop. Keep hold of the end whenever filament is unloaded; secure it to the spool before storage. Follow that guide to untangle a visible knot rather than pulling harder through the printer.

For a snapped strand, record where it broke. A hand-bend result alone does not establish the cause or rule out another obstruction. Avoid dismantling the feed path merely to prove a material diagnosis.

2. If loading fails, use the MK4 loading procedure

Prusa's MK4 loading instructions begin with a filament tip cut at approximately 45 degrees. During the normal loading sequence, the printer asks whether the extruded color is correct. If nothing emerges at that point, the manufacturer describes pushing the filament toward the extruder, then choosing Retry.

Use that step only in the documented loading workflow. If the filament remains stuck or starts grinding, stop repeating attempts and move to the relevant troubleshooting procedure. Keep fingers clear of moving parts and the heated nozzle.

The same guide says the two idler screws are factory-set. If their position was changed, consult the manufacturer's illustration: on the original MK4, the screw tips align with the front surface of the Idler Nut. Do not substitute a generic turn count or keep tightening to overcome resistance.

3. If it takes filament but does not extrude properly

Prusa's Nextruder nozzle-troubleshooting guide distinguishes partial flow from a complete jam. It also describes filament debris in feeding-gear grooves. Use the complete illustrated procedure to inspect and service the identified area.

Choose the section that matches your nozzle. The guide's needle-cleaning method is specifically for non-high-flow nozzles; it does not apply to high-flow internal geometry. For a cold pull, follow the linked model-specific procedure instead of borrowing a temperature or pulling sequence from another printer.

The manufacturer requires nozzle heating through the printer menu, not an external heat source, and warns of severe burns from hot parts. Its gearbox inspection requires a cold, switched-off printer and the specified assembly adapter. Stop at that boundary if you cannot meet the instructions; contact support instead of improvising disassembly.

4. Treat the screen message as its own branch

For MK4 firmware 6.0.0 onward, Prusa documents loadcell-based stuck-filament detection. Follow the displayed unload/reload workflow. Its detection covers pressure within the Nextruder; the manufacturer says it will not recognize outside tension from a spool tangle. No error message therefore does not clear the spool.

If detections repeat rapidly, use the loadcell troubleshooting linked from that page. If filament is present but not recognized, use Prusa's separate Nextruder filament-sensor guide. Do not disable sensing as a substitute for identifying the fault.

Confirm the result, then return to the print

Record the action taken and whether normal loading and extrusion return. Observe a short check print before committing to the failed job again. If it still fails, send support the model, firmware, material, nozzle, error code, photographs and steps already tried.

Once feeding works, use the settings hub to check the intended setup. For a material-care question, continue to drying and storage guidance; stringing with otherwise normal feeding belongs in the stringing guide. Those are related questions, not proof that moisture caused this failure.

Sources

• Prusa: Tangled filament — spool-end control and visible tangles.
• Prusa: Filament not loading, MK4/S and MK3.9/S — tip, loading retry and model-specific idler position.
• Prusa: Clogged Nextruder nozzle — partial/full obstruction and service boundaries; use the original MK4 sections.
• Prusa: Stuck filament detection — loadcell detection, limitations and repeated-error troubleshooting.
• Prusa: Nextruder filament sensor — separate presence-sensing troubleshooting route.

Sources checked October 17, 2026. This is a manufacturer-guided troubleshooting path; FilamentMap did not reproduce a jam, perform service, or test a repair for this article.