Bed Thermal Runaway
Well, it seems I'm getting the bed thermal runaway problem. I've had a few builds stop just before being done. In any case, I've tried what others suggest to troubleshoot - I set the bed to 100 degrees manually and try moving the bed/wire around. I do see the temp drop way down to 22, 29, etc. then back up to the actual temperature. It seems to stay right around 100 if the bed is still and/or the wire is not being moved.
What to look at next? My heatbed cables are wrapped in the spiral wrap with what looks like zipties holding it on both ends of the cable. Do I need to cut the zip ties?
RE: Bed Thermal Runaway
Move the bed to all extreme lengths to see if the wires move. Check to make sure the wires are not being flexed against anything.
My guess is that you have a short (break) in the wires somewhere.
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Chuck H
3D Printer Review Blog
RE: Bed Thermal Runaway
Had the same issue. This was caused by a broken thermistor wire. You should be able to isolate the defect by bending the cable partwise while watching the display. Then remove the broken part of the cable and solder a new one (or replace the whole thermistor).