MMU loading failed

Updated 3 weeks ago ​by Martin Lexa

If you see the error message “MMU load failed” on the printer’s LCD display, it indicates that the filament went through the MMU2S unit (the FINDA registers it), but it didn't reach extruder Bondtech gears. Sometimes, the filament reaches the Bondtech gears once but not for the second time. The MMU2S unit is trying to retract the filament once during the initial loading sequence to prevent possible issues during the print.

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, and your hotend and the PTFE tube(s) have to be clean (see this article if that's not the case).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 the diameter precision tolerance of only 0.02 mm compared to the industry standard of 0.05 mm. We can also guarantee its perfect ovality and provide other extra features related to the usage of the filament.

What does it look like

This is what you most likely see on the LCD screen:

You can also open the LCD menu -> Support -> Sensor info and check the FINDA value. The FINDA should be triggered, meaning the value should be 1.

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 remember the name of each part of the MMU unit. You will find this information in your printed MMU handbook as well.

There are multiple causes for this to happen. Let's go through all of them, ranged by their probability.

IR sensor is not working properly

Check that the sensor is working properly. Open LCD menu -> Support -> Sensor info and you will see the IR value. If the filament is not loaded, the value should be “0”. If the filament is loaded in between the Bondtech gears, you should see “1”. If this is not the case, perform the calibration of the IR filament sensor.

Rear PTFE holder too tight or too loose

First of all, 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.

Check also the 5 long PTFE tubes leading the filament up tu the MMU unit. See if the mmu2-rear-PTFE-holder is not tightened too much: manually push the filament back and forth through this section, it should be passing through without any resistance. The gap between the PTFE holder plate and the mmu2-pulley-body might be also too big (screws are not tightened enough), in which case the PTFE tubes might accidentally snap out.

Filament stuck in the PTFE tube

In most cases, it’s because of filament strings or the filament retracting with a thick end. 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.

To solve this, unscrew both Festo fittings of the PTFE tube connecting the MMU and the extruder. The filament string will probably stick out of MMU2S unit. Pull approximately 20 cm of filament out of the unit and cut it (the MMU gears have likely damaged part of the string, so it’s better to remove it). Make sure the filament can go freely through the tube and that the tube is clean inside. Mount everything back and resume print.

In general, if your filament comes out with a long string, it is because it unloaded at too high temperature. If it unloads with a blunt end, the temperature was too low.  Check also ther short PTFE tube that is inside the hotend. If its mouth is deformed, replace it with a new one.

Individual filament length calibration

On the MMU2S upgrade, both filament sensors (FINDA and the IR sensor) are cooperating, so there is no reason to do this calibration anymore. The length of the filament is measured automatically thanks to the new IR filament sensor mechanism.

If, however, you have changed the length of the front PTFE tube between the extruder and the MMU2S unit, you can do this calibration to ensure the filament will be loading properly. In this case, please refer to this manual.

Idler tightened on the motor shaft

First of all, make sure the motor on the left of the MMU unit (named Extruder or Idler motor) is seated correctly and in direct contact with the idler-body. The hole in the center of the idler barrel isn't entirely round, there is a flat part that matches the shape of the motor shaft. When the idler barrel is on the shaft, it has to be tightened by two M3x10 screws.

These screws must be tightened neither too much nor too little. If the screws were tightened too much, the plastic part will crack, in the opposite case, the idler will not sit properly on the motor shaft.

Check if there is a small gap between the idler barrel and the idler-body. These two parts must not be grinding one another.

FINDA check

See if the FINDA probe is correctly calibrated as described in this article. Always keep in mind the selector must be free of any leftover filament and dust.

Silent mode

Sometimes, switching the MMU2S into the Stealth mode can help. In this mode, the motors are running slower and thus the filament movement is slower. You can enable the Stealth mode in LCD Menu -> Settings -> MMU Mode [Normal] / [Stealth].

Resuming print after repair

There are two ways to do that:

  • Select Resume print on the LCD. The printer will automatically preheat the nozzle, try to load filament and if it’s successful, it will continue.
  • Press the middle button on MMU2S unit. The unit will repeat the last unsuccessful operation. If the repair was successful, the LED will flash red and green. If it wasn’t, you will see the red LED still on and you will have to try to resolve the issue again. Once you’re ready, press the middle button again.


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

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