MMU needs user attention

Updated 1 week ago ​by Martin Lexa

Did your multi material print suddenly stop and one of the LEDs is blinking red? Are the following messages displayed on the LCD screen: “MMU needs user attention.” and “Press the knob to resume nozzle temperature.”? Then the printer encountered an error during the process of changing filaments. Let's go through some of the most common causes and solutions to this issue.

Any usage and troubleshooting of the MMU2S upgrade warrant the base MK3S or MK2.5S to be in perfect shape and to print without any problem if the filament is fed directly into the extruder.

Among other things, your extruder Bondtech gears have to be perfectly centered, tightened and clean. The hotend must be clean (see this article if that's not the case) and the hotend PTFE tube needs to be undamaged and properly secured by the black collet. Failure to prevent the PTFE tube from moving up and down will cause clogging.

You also have to be using the latest version of our PrusaSlicer, have the latest firmware version flashed in the printer RAMBo board as well as in the MMU unit logic board. We strongly recommend using our Prusament filament, as it is manufactured with a strict diameter precision tolerance. We can also guarantee its perfect ovality and provide other extra features related to the usage of the filament.

What does it look like

One (or all) of the LEDs on the front of the MMU is blinking. After a minute, the extruder stops at the back left corner of the print area and you can see one of the LEDs at the front of the MMU unit blinking in a bright red (or some might say orange) light. The printer shows these two messages, one after another (the temperature may be different):

The print is thus paused until the problem is resolved by user intervention followed by a successful load or unload of the given filament, after which the LCD displays the message "MMU OK, resuming print".

How to fix it

For future reference, see the picture below and try to remember the name of each part of the MMU unit. You will find this information in your printed MMU handbook as well.

In most cases, there is no particular cause for this error. There might have been too much friction at some moment or a hobbed gear just didn't grip the filament enough. Based on the speed of the blinking, you can tell whether the filament got stuck during the loading or unloading phase.

  • Slow flashing (roughly once every second) - a problem during filament loading (the IR or FINDA sensor didn't trigger)
  • Fast flashing (roughly 4 times per second) - a problem during filament unloading (the IR or FINDA stays triggered)

Slow flashing - loading problem

First of all, try to press the middle MMU button. In some cases, if the MMU is well calibrated, it might just work. If that is not the case, you need to get to the tip of the filament. Pull the filament out of the MMU unit by winding it on the spool to prevent tangling.

If you are unable to pull the filament out, loosen the idler screws (the long screws with the spring). No need to remove them completely, just loosen them and remember how much they were tightened originally.
  1. Take the tip of the filament, cut approximately 10 cm (4 ”), and create a sharp tip (as you usually do).
  2. Insert the filament back into the long white PTFE tube hanging from behind of the MMU. Make sure the filament is able to move freely inside.
  3. Press the middle button to start the loading sequence.
  4. Push the filament in until the FINDA detects it and let the MMU retract it back to the "start position".
    1. If the LED switches to red-green flashing, press the right button to load the filament into the hotend and continue the print. (The extruder will re-heat first.)
    2. If the LED stays red, go back to step 1 and repeat the process. (If the selector moves to the right, move it back to the left by pushing the left button).

Fast flashing - unloading problem

In most of these cases, the filament stays in the orange (or white) PTFE tube that connects the MMU and the extruder. Rarely, it can be stuck in the selector or near the pulleys with a string tangled in the selector. In these cases, you need to clean the selector (see the instructions below).

  1. Remove the orange/white PTFE tube by unscrewing the silver-blue Festo fitting from the MMU selector.
  2. Pull the filament out from the tube (approximately 20 cm/ 7.8 ”), see if there is no groove or dent ground by the hobbed pulley. (The pulley may be sometimes slipping and thus it grinds the filament without actually moving it. We need to get rid of the groove the pulley creates in the filament in this case).
If you’re unable to pull the filament from the tube, unscrew the other end of the tube as well. You need to release the filament and pull it out. If needed, open the extruder.
  1. Cut approximately 10 cm (4 ”) of the filament. Create a sharp tip.
  2. Let around 3 cm (1 ”) of filament stick out of the selector. If you have opened the extruder or MMU, close it.
  3. Press the middle button and wait for the printer to move the filament back and forth. If it succeeds to do that, the LED of the concerned filament starts flashing red and green. If the procedure fails, it will stay red, in which case try to cut and load the filament again.
  4. Finally, screw the Festo fitting(s) back in and press the right button to continue the print.

All LEDs shining

In case all of the LEDs are on and the printer reports that it needs your attention, the problem is in the Selector or Idler position.

  1. Make sure the filament is out and that there is no filament in the selector.
  2. Use any Allen key to press the Reset button next to the mini USB port, located on the right side of the MMU.
  3. Wait until the printer resets the idler and th selector position.
  4. Make sure the selector is on the right place and that the correct color is to be loaded.
  5. Press the knob next to the printer's LCD screen to re-heat the nozzle and continue the print.

How to prevent it from happening

First of all, don't despair! Take a deep breath, open *this article*, read it carefully and check every aspect of the correct MMU2S setup. Afterward, continue reading below, where we'll mention a few other possible causes.
Filament tip

The shape of the filament tip is very important for a successful MMU print. The tip should be pointy, but there must not be any lump or string. The diameter of the tip can be slightly bigger than the rest of the filament, but not too much.

First of all, try to adjust the hotend temperature (increase or decrease it by increments of 2°C) which in most cases will solve the problem. You can do so during the print in the LCD Menu -> Tune -> Temperature or before slicing the model in Filament settings -> Filament -> Temperature -> Nozzle.

If the problem still persists, head to Prusa Slicer again and adjust the number of cooling moves to 2 or 3 (Filament settings -> Advanced -> Number of cooling moves), and possibly also increase the unloading speed by increments of 10 mm/s (Filament settings -> Advanced -> Unloading speed).

nacucnanej filament a vlký PVA/BVOH
IR sensor calibration

If you haven't already, make sure your IR sensor mechanism is correctly calibrated (click to see the instructions). Correct position of the chimney (IR sensor holder) on the extruder is vital to any multi material print.

Clogged selector

Always check, if the filament is not stuck behind the selector, preventing proper loading and unloading, sometimes even preventing the selector from moving.

Even a properly calibrated selector can be occasionally blocked with filament strings or dust or by a broken piece of filament. By pushing the right MMU button, move the selector fully to the right, to the service position. Use an interdental brush or any thin wire (⌀ 1.5mm/0.06”) to push the waste through (still in service position). Use compressed air to clean any leftovers.

FINDA check

See if the FINDA probe is correctly set by following the advice in this article. It must be set to the correct height and the steel ball has to move freely.

Hobbed pulleys - centered, tight, clean

The pulleys may be grinding the filament and its dust may accumulate around them. The best practice is to clean the pulleys with a mini-brush or some similar tool every now and then. The pulleys also must be perfectly aligned with the filament holes, as shown on the photo below, and well tightened by the small black grub screw against the flat part of the Pulley motor axis.

Spool slipped

Last but not least, check the filament spools. Do they roll on the spool holders without any problem? The filament should not be tangling neither on the spool nor in the space between the buffer and the spool holders.


Find out more troubleshooting tips here: Community Forum | Assembly Manuals | Youtube channel |

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