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MMU Needs Attention after every print.  

Eminent Member
MMU Needs Attention after every print.

After every print, I get the "Error, MMU needs user attention. Press the knob to resume nozzle temperature".

If I unload the filament (manually), hard reboot, reload the filament, it will do a perfect print, but then return to this error after the print.

I'm not sure how to start troubleshooting this, any tips?

Posted : 02/08/2021 8:09 pm
Eminent Member
More detail needed

Can you give more detail:

  1. Is this on multi-material prints or using the MMU for single material prints?
  2. What filament are you using?
  3. What mods did you make for the printer? Are you using the buffer?
  4. How do you store filament?
Posted : 04/08/2021 2:55 am
Eminent Member
Topic starter answered:
Here you are


Posted by: @gglockner

Can you give more detail:

  1. Is this on multi-material prints or using the MMU for single material prints?
  2. What filament are you using?
  3. What mods did you make for the printer? Are you using the buffer?
  4. How do you store filament?

Sure thing,

1. MMU for single prints

2. 3D Fuel PLA+ 1.75mm

3. I am using the buffer, and I printed the mods that add the bowden couplers both to the front and back of the buffer, as well as the back of the printer where the bowden tubes enter.

4. In a plastic bin, but right now on the spool on top as I made the Ikea enclosure.


Thanks 🙂

Posted : 04/08/2021 5:08 pm
Eminent Member
  1. I have had far more extruder jams with PLA than with PETG. Try printing something with PETG to see if you still get filament unloading jams in the extruder.
  2. In my (limited) experience, extruder jams depend on the quality of PLA. If the PLA is very brittle, it's much more likely to jam on unloading. Make sure your PLA is very dry. Also make sure to use the Prusa slicer filament profiles.

Keep us posted.

Posted : 04/08/2021 5:37 pm
Reputable Member

Is this with prints sliced for MK3 MMU Single or for a normal MK3? You can print gcode sliced for a normal MK3 if you use "Load to nozzle" before you start the print. I have never done that and I'm not sure if the MMU will unload the filament after the print finished.


What is the MMU trying to do when you get that error? Is it trying to unload, but fails or has it unloaded from the extruder already?

Posted : 04/08/2021 6:10 pm
gglockner liked
Reputable Member


Posted by: @baklin

... I have never done that and I'm not sure if the MMU will unload the filament after the print finished....

It will not unload but also will not generate an "MMU needs attention" error. It just stops like it would if no MMU was installed...

If at first you don't succeed, skydiving is not for you.

Posted : 31/08/2021 12:15 pm
Active Member
Same issues too!

Mine started doing this, just as I replaced the hot-end on mine. I couldn't figure out what the flip was going on.

Things that have partly resolved mine:


Took the extruder assembly apart, and made sure everything was clean.


By take a part, I just mean, remove the filament fan, the V6 fan, and pull the motor back, you don't need to take the carriage off or anything.

Check the extruder gears, make sure the teeth don't have filament in them, as the extruder does most of the initial pulling.

Put it all back together, but left the filament tension part off. 

If you have the 2mm hex key, or what seems to be the perfect size and strength for me, is a wire hangar. Clip that bad boy about 12 inches long, straighten out as much as you can. Then, pre-heat your machine to 240c or so, (although not recommended) turn off the machine,  then gently, from the top of the extruder where your Bowden tube goes in from your MMU, remove the tube, and push the metal hangar down the hole, as if it were filament. You shouldn't have much, if any resistance because the other part of the extruder is off the machine, meaning if there's no issue, your wire/hangar should go all the way to the tip of your hot end. You might feel some slight resistance when you get there, but if you look, there's probably some excess filament coming out.

I pull up and down a few times, make sure it's all good. So now you've made sure there's 0 bockages from the extruder pully to the hot-end, 


Now that you got that sturdy ass wire or rod or whatever, check your alignment to the gears. You want to have that center so when the filament comes down it doesn't really have to be pulled, rather gently tugged.

If you got all that good, throw on the filament door, and tighten the tension screw so that the screw is just flush with the nut facing you.


While the machine is still off, you can take that rod and GENTLY push down and up to get those gears to line up, and you should see both of them moving. 

You can then use this time to tighten that long ass screw in the back that tightens to the extruder motor. (too tight and that door won't move as freely as it needs to, giving filament a hard time to go down and come back up.

After all that's done, boot up the printer, run a pre-heat to pla or whatever filament you got, and try a "load filament to nozzle".

If it all goes well and it gets to the nozzle and you get to select "did it come out the right color" or whatever, I just hit "No" a few times, then I send the "unload filament" command.


Listen for any grinding, might be a sound of skipping filament or too tightly on that filament door.

If you're hearing that amazing whine from the MMU but the filament isn't moving, well congratulations the MMU is dirty AF and probably needs cleaning and proper tensioning.

Similar to the extruder, you can feed that hangar wire through each of those holes, make sure that each of those gears lines up center to the hole, then tighten them back down.

As a lazy mans cleaning. I usually just leave the MMU top open, and tell it to load filament to nozzle while all the filament is out of the mmu and just put a wire brush over those gears so it can get the junk that builds up over time. 

I notice different filaments seem to flake and get in those grooves more than others.

Posted : 10/09/2021 4:04 pm
Active Member
Clean filament path


I had the same problems some weeks ago. The main problem was a small string of filament in the PTFE tube of the changer. This string seems to cause trouble with the detection of the PINDA sensor in the changer. After removing the string, the problem was gone.

As mentioned earlier, PLA could cause some issues and the printing temperature might be a little bit too high (causing stringing). So it might help to reduce the nozzel temp a little bit for the last layer. PETG and ABS are also causing less problems with filament change. I printed several projects with ABS/ABS+ with more than 200 toolchanges without any problems.

Posted : 17/09/2021 6:57 am