The MMU3 does not load, does not unload or the filament gets stuck

Symptom

The MMU3 stops at a colour change and the printer shows an error, for example «FINDA didn't trigger» (#04101), «FINDA: Filament stuck» (#04102), «Filament sensor didn't trigger» (#04103) or «Load to extruder failed» (#04108): the filament does not reach the FINDA sensor in the MMU, does not reach the extruder sensor, stays inside when unloading, or the loading check fails.

Likely causes, from most to least likely

1. The filament tip is deformed, bent, stringy or thicker, or the last 40 cm have gear marks, so it does not pass properly through the tubes and gears.
2. Too much friction or a tangle on the spool or in the buffer, or the MMU pulley does not manage to grip the filament.
3. The PTFE tube between the MMU and the extruder is not fully pushed into its (Festo) fittings or its length is not right.
4. The extruder filament sensor needs recalibrating: the loading check is very sensitive.
5. There is debris or pieces of filament in the selector or in the extruder idler, or the FINDA sensor is not working properly.
6. The filament settings do not match the material, or the filament is damp or of poor quality.

Checks to tell which one it is

• Look at the error number: #04101, the filament did not reach the FINDA; #04102, the FINDA still detects filament after unloading; #04103, it did not reach the extruder sensor; #04108, the automatic loading check failed.
• Pass a piece of filament through the FINDA and check whether its state changes in Info → Sensor Info (MK4, MK4S, CORE One) or in Support → Sensor Info (MK3S+).
• Release the PTFE tube, pull out about 40 cm of filament and check the tip and any marks.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Prepare the filament

   Cut off the deformed or bent tip so it is straight and sharp. Check the last 40 cm: if they have gear marks, cut them off. Make sure the spool turns freely and there is no friction or tangle between the spool and the buffer.

   Check: The filament goes in and out without snagging and loading finishes.

   If that does not fix it: Check the PTFE tube (next step).

2. PTFE tube and fittings

   Check that the tube between the MMU and the extruder goes all the way into the fittings at both ends and that the fittings are fully tightened, with no gap. If you replaced it with another tube, check that its length is within the range Prusa specifies. On the CORE One, enter the tube length in Settings → Hardware → MMU.

   Check: The tube does not come out when pulled gently and the filament passes cleanly.

   If that does not fix it: Recalibrate the extruder sensor (next step).

3. Recalibrate the extruder sensor

   On the MK4, MK4S and CORE One, check that the extruder has the MMU modification, run Gears Calibration and Filament Sensor Calibration from Control → Calibrations & Tests and try loading manually with Filament → Load to Nozzle. On the MK3S+, recalibrate the extruder IR sensor following Prusa's MMU3 guide and use Settings → Loading Test with the printer idle.

   Check: Loading to the nozzle completes and the extruder sensor detects the filament.

   If that does not fix it: Clean the selector and the extruder (next step).

4. Clean the selector and the extruder idler

   With the printer off and cold, open the fitting on the selector and remove any debris or broken filament. Open the extruder idler and remove any leftover pieces.

   Check: The FINDA stops detecting filament when there is none and the path is clear.

   If that does not fix it: If the FINDA still does not respond, follow Prusa's FINDA setup and troubleshooting guide.

5. Check the MMU mechanics

   If the filament does not reach the FINDA, check that the pulley grips it (if not, it is too far back), tighten the two spring screws on the sides of the MMU properly and check that the barrel is oriented as in the manual and that the pulley gears are clean, free of filament bits and aligned with the filament.

   Check: The filament reaches the FINDA without stopping short.

   If that does not fix it: Check the filament settings and quality (next step).

6. Filament settings and quality

   Check that the PrusaSlicer filament profile matches the loaded material and that the nozzle temperature is right. Dry the filament if it has absorbed moisture and use good-quality filament.

   Check: Colour changes complete without errors during the print.

   If that does not fix it: Contact Prusa support and give them the error number.

Prevention

• Keep the filament dry and check that the tips come out sharp on every unload.
• Keep the spool and buffer free of friction and tangles.
• Recalibrate the extruder sensor after disassembling or repairing the print head.

Still not fixed?

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Printers it applies to

Prusa MK4, MK4S, MK3S+ and CORE One.

Official sources

This guide summarises the official documentation linked here:

• FINDA didn't trigger #04101 (MMU) · Prusa Research
• FINDA: Filament Stuck #04102 (MMU) · Prusa Research
• Filament sensor didn't trigger #04103 (MMU) · Prusa Research
• Load to extruder failed #04108 (MMU) · Prusa Research