I have a Prusa MK3s MMU2s which I was running a 7-hour print job on.

It had two spools of filament in, a blue one (primary, very little left) and a gold one (secondary, quite a lot left). It ran out of blue filament approximately 75% through the print job and rather than simply using the remainder of the blue and switching to the gold, the blue was bent and caught on the output nozzle of my spool container. It says that:

MMU needs user attention, fix issue then press button on MMU unit.

I have fixed the issue (snipped the bent part of the blue filament), however, I can't figure out how to make it continue printing. After fiddling a little bit, the red and green lights are all flashing, but no matter what MMU buttons I press, it doesn't continue printing. How should I fix this without cancelling the print job?

  • $\begingroup$ I assume you looked up the issue on Prusa's web site? $\endgroup$
    – BobT
    Apr 9, 2021 at 21:23

1 Answer 1


This condition is covered in the MMU2S manual's section on troubleshooting:

13.4 All five LEDs blinking red and green

MMU2S unit can now deal with a state in which the Trinamic drivers were not able to provide enough power for steppers. A possible cause can be a broken MMU2S power supply wiring, connectors etc. Please check that all connectors are properly plugged into the MMU2S board and make sure that the cables are in good condition. Also, make sure that the screws on the Einsy terminal box for the power cables are properly tightened. If this state is detected, the MMU2S unit rehomes and automatically continues printing. If the problem occurs three times in a row, the MMU2S unit stops printing and starts flashing continuously with red and green LEDs. Printing can be restored by restarting MMU2S unit using the button on the right side of MMU2S unit.

So, once you have fixed the problem which prevented the filament from moving, press the reset button on the MMU2S. It is recessed, so you will need a tool to push it (such as the 2.5 mm Allen key included with the printer). You will then need to wait a few seconds for the MMU2 to complete its reboot and be ready to communicate, before telling the printer to continue.

Note that the filament may have been chewed by the MMU2 or extruder drive gears and unable to feed properly. Be prepared to — if you haven't already — disassemble the tubing and manually pull out the remaining filament sections. I find this modified holder for the MMU2's filament tubes that uses push-in couplings instead of a clamp very useful to allow quick access when there's a problem or even when inserting new filament.

  • $\begingroup$ I ended up just resetting the printer. The issue was a bend in the filament, which would have been a lot of trouble to fix (being inside the MMU), especially for a comparatively short print. $\endgroup$
    – nerdguy
    Apr 10, 2021 at 1:02

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