Sensor faulting faster than LCD can display
This may help someone or not, but I thought I would share.
I was getting repeated MMU2 faults that would occur while printing (vs loading). It would go right to ejecting filament. I thought it was the IR sensor at first.
I put together a quick LED circuit to show the logic level on both sensors. Once that was working I noticed the LED for the FINDA sensor was showing the logic dropping out when the filament moved a certain way but it was not changing status on the LCD screen unless it was held in error for a length of time.
The adjustment was really sensitive, but after making sure it indicated properly, I re-ran the print and got the 6 hour 3 color lizard done without a single MMU fault, but it did have a skipped layer.
I am still running 3.6 and plan to load 3.7 next.