MMU3 can't be enabled for Core ONE unless side filament sensor has been calibrated, even though it isn't needed.
Printer: Core ONE
Firmware: 6.5.3+12780
I was recently printing with a filament that doesn't work well with the MMU3, so I had it disabled and was feeding filament in through the side input. When I was preparing to switch back to using the MMU, something got jammed in the side filament sensor, and it was stuck reporting that there was filament present when there wasn't. I tried to recalibrate the sensors, but it failed. I decided to switch back to using the MMU again at that point, but I couldn't re-enable it in the printer settings because the filament sensor calibration had failed. I ended up having to take off the side panel and disassemble the side filament sensor to clear the jam before I could run a successful calibration and finally re-enable the MMU.
To reiterate, I could not enable the MMU in the printer settings because the side filament sensor calibration had failed, even though the fact that I am using the MMU means that I don't need the side filament sensor at all. I had to partially disassemble the printer to fix a hardware problem completely unrelated to the MMU before I could use the MMU.
I'm assuming the calibration check is in place because it's risky to run the MMU3 when the toolhead filament sensor is not reporting accurately. I'm also assuming the calibration check is just checking the result of the Filament Sensors step of the Calibrations and Tests menu as a whole, and failing one of them them fails the whole thing. That would mean the two sensors need to be calibrated separately. Although now that I say that, I'm not sure what calibration actually needs to happen for the side filament sensor. It seems to be pretty binary, unlike the toolhead sensor. Maybe it doesn't need to be checked at all?