[BUG] MK4S + MMU3 stuck at “Reloading filament 99% (Parking)” during single-material print

[blair-ko]

Printer model

MK4S

Firmware version

6.4.0 (also 6.4.0+11974)

Upgrades and modifications

MMU3 attached

Printing from...

Prusa Connect + Prusa App

Describe the bug

During a single-material print with MK4S + MMU3 on firmware 6.4.0, the printer unexpectedly enters a "Reloading filament – 99% – Parking" state even though no filament change occurs. In this state, there is no filament movement, the extruder motor does not turn, the toolhead is parked, and the UI becomes unresponsive. The print does not progress and remains stuck indefinitely until the printer is reset or power cycled. This bug also leads Prusa Connect/App to incorrectly detect the printer as idle and issue an automatic cooldown, shutting down the hotend during an active print.

How to reproduce

1. Start a single-material print on MK4S with MMU3 using firmware 6.4.0 (or 6.4.0+11974).
2. During the print, the printer unexpectedly enters a "Reloading filament – 99% – Parking" state.
3. In this state, there is no filament movement; the extruder motor does not turn; the toolhead is parked; and the UI is locked (rotary knob only beeps).
4. The print remains stuck indefinitely until the printer is reset or power cycled. Prusa Connect/App will falsely detect the printer as idle and attempt to shut down the hotend.

Expected behavior

The printer should never enter a reload state during single-material prints when filament is already present. It should continue printing the G-code as normal. If a reload is attempted, the firmware should provide a timeout or fallback and allow the user to cancel or retry via the UI. The UI should remain responsive, and Prusa Connect/App should not shut down the heaters while an active print is running.

Files

MK4S_MMU3_bgcode_images.zip

[m42e]

This is the same: #4791

[wseifert]

Just had the same issue with my MK4s + MMU3: after 3 layers printer stopped, moved head to right front and up and on printer display was stated >waiting for filament reload<. No further action occured. Had to reset printer. After restart the print with same g-code job was successfully done.

[tomisl]

The same situation. Don't eve start print. 6+ times. Revert fv to previous version and works.

[cyrilknops]

Same issue, mk4s with mmu3. Tried it multiple times with the same g-code. Every time it happend around 80%, keep in mind it was just a small print.

[Prusa-Support]

Thanks for reporting.
Would it make any difference if the feature stuck filament detection is disabled or the firmware is downgraded to 6.2.6?
If so, we may be looking at a known bug, and a bugfix should be in progress.

Michele Moramarco
Prusa Research

[wseifert]

Did not have the issue for long time, it is sporadic so I can wait for a fix ...

[iambighead]

Thanks for reporting. Would it make any difference if the feature stuck filament detection is disabled or the firmware is downgraded to 6.2.6? If so, we may be looking at a known bug, and a bugfix should be in progress.

Michele Moramarco Prusa Research

some feedback here

I compiled a custom firmware based on 6.4.0 + the patch from #5082

With the patch, the "reloading filament 99% (parking)" issue is gone, but instead now it became filament stuck and went into filament reloading procedure, not ideal but at least I can recover and resume the print.

Then I tried disabling "stuck filament detection" and the issue is totally gone (at least so far), all prints are completed without issue

So the way i see it there are 2 issues:

1. the stuck filament detection might be too sensitive, maybe there is a way to calibrate it to make it less sensitive?
2. in case of a genuine filament stuck issue, the printer/firmware should allows user to reload instead of stuck at showing "reloading filament 99% (parking)"

cheers