I have a i3 MK3 and ran into a bed thermal runaway Error today. I have reflashed it, and continued to get the error message. After looking into it a little further, it looks like the wire of the thermistor of the bed has had some extra wear exposing the wire. I believe this is what is causing the heat bed to shut off. I was curious to know what you believe the course of action is for fixing this problem.
RE: Thermistor replacement?
A broken heatbed thermistor wire would normally be seen as an open circuit and trigger a mintemp error.
A bed thermal runaway error would be triggered by the firmware setting the bed to heat but not detecting the expected bed temperature rise within a reasonable period afterwards. The causes of this error message are normally a broken heatbed cable connection or a themistor that is no longer in good contact with the heatbed.
Check that your heatbed thermistor is still firmly and directly attached to the back of the heatbed for good thermal conductivity.
Disconnect your heatbed from the einsy board and use a multimeter or continuity tester to confirm that the heatbed heating cables are good. If they show an open circuit or high resistance or intermittent electrical connection check the connection at the heatbed side to determine if the issue is the cable or the heatbed itself.
Also check the heatbed fuse (the 15Amp fuse) installed on the einsy board; this fuse will blow if there is a short on the headbed power circuit. If this fuse has blown, find the cause of the short and fix it first before you replace the fuse!