Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the blue wires from the return thermistor

Measurement:

Measure resistance between one of the thermistor terminals and its retaining screw (it does not matter which terminal or screw).
Expect open circuit / O.L.
Is the resistance measured as expected?