Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect connector PL1 from the PCB

Measurement:

Measure the resistance across the two yellow wires at PL1.
Resistance will vary according to the temperature of the water.
The lower the temperature the higher the resistance.
The resistance should conform approximately to the table below:
Temperature °CResistance in kOhms
20-301050-780
30-40780-600
40-50600-450
50-60450-260
60-70260-150
70-80150-55