Set Appliance State:

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

Measurement:

Measure resistance across the two yellow wires
The resistance measured will depend on the boiler temperature.
The hotter the boiler, the lower the resistance.
Temp °CResistance in kOhms
20 - 308 to 16
30 - 40 6 to 10
40 - 503.9 to 8
50 - 602.5 to 5.5
60 - 701.5 to 3.0