Thermistors change resistance with temperature (either up or down with heat, depending on type). If you have an ohm meter you could monitor the resistance on the thermistor as the heater engages and starts heating. If the resistance doesn't change, something happened to the thermistor wiring (very unlikely that the device itself went bad).

I just finished building two new E5 Pro units. Both exhibit the same behavior - about 2 seconds after bed heat is applied (either through Control or Prepare menu) the controller roboots. This sets the heat setting back to zero.

The nozzle heats up fine but as soon as I attempt to heat the bed, the reboot occurs and the nozzle heat gets set back to zero.

I swapped the beds between the machines. I've double checked wiring. I can't help but think I've done something wrong. Two units doing the same thing.... Hmm.

As a side favor, could someone point me to where the manual lives? I've looked all over the site and can't locate it. Feeling kind of dumb now.