hotpoint wm62 not heating up?

My hotpoint wm62 washing machine wil not heat up the water on any of the selected temperatures.
The machine will go through the whole cycle without any glitches.
I have checked the heater with a insulation tester which puts out 500v and no shorts or breakdown in the element is showing.
I also disconnected the thermistor and machine worked as previously stated.
Do you think the thermistor, which is part of the heater element assembly is at fault or the main board?