Printer: Max V2, Prometheus hotend, stock "Grain of wheat" thermistor, Duet Ethernet controller.
My printer was happily printing away when I heard a loud bang and the temps on both the hotend and the bed went to 2000 degrees.
Here's the symptoms and how I troubleshot this problem:
I took the board out and looked for burnt/blown components, nothing. No shrapnel from a popped chip either. So far, so good.
The temps would display fine if the power to the controller was off and the board was being powered up from the USB cable. Okay, so something's shorted to the power rails.
Unplugging the hotend thermistor (which is the one that typically is going to fail) and turning on the controller showed the bed temp working, and the hotend temp at 2000 (open circuit on the thermistor).
Plugging the bed thermistor into the hotend connector showed the proper temperature there too. This meant that the controller had most likely survived (Yay!). Ohming out the hotend thermistor showed about 88K ohms, which was about right given that the hotend hadn't cooled off as of yet.
So it would appear that everything was fine. But there are other potential paths for electricity available, so not really. Leaving the hotend where it was, I checked the resistance between the thermistor and the +12VDC and Ground from the power supply. No surprise, one of the thermistor leads was shorted to +12VDC.
Moving the carriage to remove the hotend "fixed" the problem. That is, until it happens again. Not much of a "Fix". So I pulled out the stock thermistor and screwed in a replacement that comes with a threaded end that screws into the hotend block.
Postmortem investigation showed that one of the leads to the old thermistor had its heatshrink tubing pushed back, exposing the tiny lead. It appears that it made contact with the heater cartridge. But I didn't see where the leads were burnt/melted from the short.
Lessons learned:
I'll install my thermistor, which will be the screw in type from here on out, on the opposite side of my heater cartridge.
And high praise for the Duet board for shrugging this off!