MMU2 LEDs meaning

Updated 2 months ago ​by Martin Lexa

As you can see, there are five main LEDs on the MMU2 unit. Their arrangement corresponds to the arrangement of the 5 PTFE tubes and 5 filament channels. Each LED consists of two sub LEDs: Red status LED Green status LED.

Green light

As you would guess, the green light means that everything works as it should.

  • Stable green light (not flashing) - selector moved to current position and ready.
  • Stable green light after loading - filament successfully loaded
  • Rapid green flashing light (after pressing the middle button) - loading the filament sequence
Note, this sequence won't stop until FINDA senses the filament flowing through or the user cancels the operation pressing the middle button.

Red light

As for the red light, its meaning also depends on the context:

Stable red light (not flashing) 

Wait until the printer finishes the current activity (there is not necessarily a problem).

Slow flashing during print 

An error occurred during filament load.

  1. Pull the problematic filament out from the PTFE tube and cut the end to create a sharp tip
  2. Insert the filament back into the white PTFE tube
  3. Press the middle button to start the loading sequence
  4. Push the filament until it’s caught by the drive gear
  5. If the LED turns green, press the right button to continue the print
  6. If the LED turns red, repeat the process.

Rapid flashing during print 

An error occurred during filament unload.

  1. Remove the orange/white PTFE tube by unscrewing the silver fittings from MMU2 body
  2. Check whether there’s a filament string stuck in the MMU2’s selector
  3. Pull the filament out from the orange/white PTFE tube
    1. If you’re unable to remove the filament from the tube, proceed to the other end of the tube
    2. Release the Festo fitting on the extruder and if needed open the extruder idler
    3. Then pull the filament out from the extruder body
  4. Using your fingers move the filament back in the MMU unit and leave about 1 cm sticking out.
  5. If the filament is stuck inside the selector and you’re unable to move it, press the left button - this will start load/unload
    1. If the printer failed at the loading stage the button works in loading mode
    2. If the printer failed at the unloading stage, the button is in unloading mode
    3. The middle button will be used now. Press it once and wait for the printer to finish the auto-check procedure.
  6. If the procedure finishes correctly, the LED above affected filament will turn to red-green flashing, if the procedure fails it will remain red.
  7. To continue after fixing the issue, press the right button.

All five LEDs blinking red after power on

If the filament is present in the selector during power up, it’s detected by the FINDA and the standard booting move sequence is interrupted, as it would bend or break the filament. Remove the filament manually first so the boot can continue. No need to press any button. As soon as you take the filament out, the printer will continue by itself.

  1. Try to pull the problematic filament out from the back.
  2. If you’re unable to pull the filament out, try powering the printer off and try again.
  3. You can unscrew the FESTO coupling from the MMU2 selector and instead pull the filament out from this side.
    1. If you’re able to pull the filament out in this direction, cut it at near the spot where the filament enters the white PTFE tube.
  4. As a last resort, remove the selector motor by unscrewing the 3 screws holding it and push the smooth rods out with an allen key. You can then easily remove any debris stuck in the selector.


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Find out more troubleshooting tips here: Community Forum | Assembly Manuals | Youtube channel | info@prusa3d.com


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