Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the 3 pin connector marked J5 (yellow leads) from the PCB

Measurement:

Measure resistance across connections 2 and 3 (yellow) leads at the disconnected connector.
The resistance will vary according the boiler temperature but should conform to the table below:
Temperature °CResistance in kOhms
20-301000-1400
30-40 400-675
40-50235-400
50-60144-235
60-7070-80