Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap - Hardware, firmware and software help

RE: Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

ViewtifulSchmoe — Tue, 05 May 2026 14:42:07 +0000

I tried this late yesterday. Funnily enough, as soon as I turned the MMU back on from the LCD screen, the unit came to life, homed, and now appears to be ready to go.

I have a few more prints I would like to finish in single-material mode before I try printing with the MMU again. So, I will be back in a couple of days with a final report. Thank you both for your assistance so far.

RE: Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

miroslav.h4 — Mon, 04 May 2026 07:26:33 +0000

On the printer display, set the FINDA sensor status monitoring to some position in the bottom information line. And then repeat the attempt by inserting a piece of filament into the MMU FINDA sensor and watch whether this data changes (ON/OFF) in accordance with the red indicator light on FINDA. If not, check the FINDA cable for damage.

RE: Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

mnentwig — Sun, 03 May 2026 21:02:03 +0000

BTW a stepper taking damage from mechanical overload would be surprising - usually they are driven "constant current" so power dissipation on mechanical stall does not skyrocket (as it would e.g. for a brushed motor)

RE:

mnentwig — Sun, 03 May 2026 20:48:29 +0000

>> When powering on, it does not attempt to home at all

Usually this is stuck filament in the FINDA but ...

>> I have verified that the FINDA sensor is not detecting any stray filament by inserting a small piece of filament into the sensor. When I insert the filament, the red LED on the FINDA turns off. When I remove the filament, it turns on.

... you already confirmed that the LED on the sensor responds but the MMU doesn't react. This looks to me like the signal might not be getting through. The MMU3 will not try to initialize with detected filament (as it would get stuck).

Can you check the FINDA cable and the connector? Is it possible the connector is missing one pin (this can happen e.g. when it gets bent slightly in handling then crushed flat by the connector)

RE: Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

ViewtifulSchmoe — Sun, 03 May 2026 19:58:08 +0000

I have not yet figured it out. I reached out to support late yesterday. Hopefully they respond Monday or Tuesday.

RE: Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

Jeff — Sun, 03 May 2026 17:20:22 +0000

Did you find a solution to this? I have almost the exact same symptoms and sequence proceeding the failure.

Enclosed MMU3 on Core One has suddenly become nonfunctional after a filament swap

ViewtifulSchmoe — Sat, 02 May 2026 18:14:54 +0000

The MMU3 on my Core One has become nonfunctional after changing the filament. It completed several small prints yesterday, so I don't know what happened. Here is the sequence of events immediately before the unit stopped working:

I pulled all filament out of the MMU3 and the filament buffer manually.
I loaded new filament into slots 1 through 4 by following the "Preload to MMU" prompts.
I used the "Cut filament" prompt to check that filament 1 was properly loaded. I did this because the loose filament in the buffer repeatedly got caught on something as I put the cartridge back into the buffer.
I hit the reset button because the MMU3 got caught in a loop trying to cut filament 1, indicating that the filament had become partially unloaded (as I had expected).

Since then the MMU3 has been completely nonfunctional. When powering on, it does not attempt to home at all. There are no motor noises of any kind. The filament selector stays on filament slot 1, and the LED for slot 1 is lit green. The filament selector buttons yield no response. The MMU3 reset button causes the unit to flash lights as if rebooting, but then it goes back to the same state: stuck on filament slot 1 with the slot 1 LED lit green. At no point does the Core One's LCD screen indicate that an error has occurred.

I have taken the following steps to troubleshoot / resolve the problem:

I have verified that the FINDA sensor is not detecting any stray filament by inserting a small piece of filament into the sensor. When I insert the filament, the red LED on the FINDA turns off. When I remove the filament, it turns on. There are no other changes to the MMU's state.
I have re-flashed the MMU3 with the latest firmware. PrusaSlicer indicated that the firmware flash was successful. However, upon rebooting, the MMU3 returned to that same state: stuck on filament slot 1 with the slot 1 LED lit green.

I am inclined to think that the filament selector motor has failed, but I want to rule out other possibilities before I contact support for a warranty replacement. What other troubleshooting attempts should I take?