MINTEMP BED and Fan Self-Test Issues Solved
I wanted to put a few notes down to help others who may encounter similar errors.
After about ~100 hours of printing on two of our Prusa MK3 printers (a B3/R2 version, and a B7/R3 version), I began experiencing very intermittent MINTEMP BED failures. Sometimes, a 24hr print would be fine. Other times, it wouldn't make it past initial bed measurements. I tried most of what was recommended in various places: firmware up/downgrades, factory reset, checking RAMBO wires are connected, moving the bed around to throw the error (with no luck), etc.
I eventually gave in and took the bed off to see if it was actual hardware. The thermistor was still properly soldered and I was able to repeatedly measure the resistance on the copper wire (after stripping insulation near it for visual inspection). However, I noticed that I would get intermittent resistance values when measuring from the connector-side of the cable if I wiggled the cable. Both of our printers experienced wire breaks/failures inside the insulation near the bed feed-in strain relief. I had a spare thermistor on-hand, so a simple part replacement solved my issue for one of them; ordering another thermistor soon.
During the factory reset, I was getting fan self-test failures on the B7/R3 version when the extruder was up against the left side of the bed. I discovered a similar issue, where one of the fan cable wires had broken inside the insulation at the top of the RAMBo feed-in strain relief. I didn't have a spare fan, so I found the specific break location and which wire it was, re-soldered, and wrapped it in Kapton tape. A replacement fan will be ordered soon.
Good luck!