Failure to load to extruder.
I'm having an issue with my MMU3 where at some point during a print (and prints are always long with the MMU), it starts failing loading to extruder. I'd say 90% of the time this is death to the print, as the firmware goes into a loop and you can't do anything about it.
If I unhook the bowden tube at the extruder, the filament will just push and push. So I know that's working. When it hits the extruder, the behavior changes (it starts to go into loading mode), so I know the filament runout sensor is working. The behavior changes.
Then it unloads back to the MMU, and then loads again. Over and over and over again until it errors out, and then the only option you have is to click the button and try again, or just turn the printer off and give up.
In the image - the little boxes on the LCD. Boxes and Dashes. They appear to represent something, as they change... like possible trying to tell me the filament sensor is going off and on again...? but I can't find anything in the documentation to indicate what this means.
I'm debating on hanging up the MMU until nextruder support is released. Which would be sad, because I hung up the MMU2s because of the never ending power panic issues. That's fixed, but now the filament sensor is doing something wrong.
RE: Failure to load to extruder.
I had the issues with the MMU2, now with the MMU3 I have much better success. On the MMU2 it was all about the calibration of the IR Sensor. That was a pain in the b.. on the MMU2, and the MMU3 with the new tower makes it easier. Calibrate again, make sure it is working really well and consistently.
Take a look how the beginning of the filament while it is retreating from the hotend. It should have a small tip. On my initial runs it was squished flat and it wouldn't reload. I reduces the pressure on the idler door. Make sure the screw through the idler door (the one with the spring) isn't to tight (or to loose).
The mechanical filament sensor in the MMU seems to work really well. Why to we need to hazzle with the IR sensor? Can't this be replaced with an mechanical sensor?
not sure if any of this helps
good luck
Wolfgang
RE:
Hi Wolfgang,
MMU1 didn't have any Filament sensor, but if well set up. It worked
MMU2 was fitted to the Mk3 printer with the laser filament sensor.... the laser filament sensor was not good for the MMU2, so Prusa changed from Mk3 to Mk3S and the MMU Became MMU2S, the mk3 had the new infra red filament sensor,
some infra red sensors gave issues. others didn't... the Infrared Filament Sensor was awkward to set up, with finess... so I modified mine.
As you can see, I Added a 5volt LED between the 5volt pin and the Signal OUT Pin and I added a hole on the extruder cover for it to show through. this gave instantaneous confirmation of functionality
for the MMU2, the filament sensor was in a tower, however the modified filament sensor would work there as well
Again this gave instantaneous confirmation, and made filament sensing easier to set up.
the first filament sensor tower was a bit difficult to set up, so MMU3 comes with a new filament sensor tower, and mine still has the modified Filament sensor.
I Find this invaluable for discerning errors,
for instance, if you have a bit of a blockage which causes the filament to get ground away a little,
You will be able to see the filament sensor see the filament when it is next inserted, but there will be a flicker of the LED, as the won part passes the sensor. this is enough to trigger a filament out situation. even though there is filament in the extruder... this is something that you will never see, on the sensor screen on the LCD. unless you happen to be looking at that screen, at the exact moment that the damaged filament passes the sensor
we are all makers, there is nothing stopping you designing a better filament sensor, many improvements start from Community experience. so far, I have not given enough thought to a better filament sensor, because My LED adaption has worked well enough.
I am however on my third Finda. the first two have gone unreliable. possibly due to the cable entry, into the Finda Body, having little strain relief.
I found this one available on Ebay,
https://www.ebay.co.uk/str/tcmuk3dltd?_trksid=p4429486.m145687.l149086 
It is intended for the Prusa Mk2 printer, so it has the correct plug for the MMU, the cable is a little long but you can cable tie that out of the way
It is currently working on My MMU3
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Failure to load to extruder.
Make sure the extruder mounting bolt that holds the idler door isn't too tight. With the idler screw removed, this door should move freely. This caused me a lot of problems at one point.