My heated bed had intermittent issues with losing temperature which I thought was caused by a damaged wire. I have now replaced the whole ribbon cable twice but now I cannot get any temperature. The reading is always 0 degrees. I have checked the resistance across the thermistor and get 100k. I have checked all of my soldering and cannot find a fault.
Are there any other checks that I can do to narrow down the fault?
Try measuring at the solder joints on the bed to see if you still get 100K ohms if so check at the connector (wires) to see if the 100K is still there.
If it is try swapping therm 1 and therm 2 on the control board to see if the problem moves.
I had the same problem. For me the +12V ripped off the heated bed.