I sent this same question to tech support, but thought I would also ask the community here.
Recently I had a print on my Mini halt after several hours, with the error “Thermal Error, system stopped! Heater_ID: bed.” The next print then ran for several hours before failing with the same error. Trying two more prints it happened within the first two layers/few minutes. Screenshots of OctoPrint’s temperature readings in those latter two prints are attached. A picture of the bed and print at the point of the second failure is also attached.
There were no software, hardware, or other changes made before these faults began, the printer had been functioning quite well.
Thermistor cable connectors at both the end of the cable harness and on the control board seem to be properly seated. Measuring the thermistor’s room temperature resistance at the connector yields 110K. I do not notice any bed temperature variations at any of several temperatures if I just let the printer idle, but I also don’t see any if I manually direct the bed to move back and forth repeatedly. There is more variability in the reported bed temperature then I had ever noticed before, but no dramatic instantaneous spikes like I’d expect with a broken wire (i.e., in the cable harness). In one case it does seem like there was a somewhat quick drop in the reading, but in the other there’s a gradual downturn for several minutes before the fault.
The PEI sheet on this printer was replaced earlier this summer (and thus the cables disturbed), but a substantial amount of printing has been done since then. One potentially related note though is that at some point fairly recently the new sheet started to scratch and scuff when removing pieces, which in hindsight was perhaps related to not actually being at the release temperature due to faulty readings? I have not though had problems w/ parts sticking to the bed or otherwise had reason to believe it was not being heated properly.
Update: Resistance on the heating pad itself as well as the pad & thermistor wires in the cable bundle to the RAMBo all also check out.