Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the brown and blue leads at the inline connectors from the flue temperature thermostat

Measurement:

Measure resistance across the exposed sensor terminal leads.
The resistance depends on temperature and should
conform to the table below:
Temperature °CResistance in kOhms
20 -3010 - 8
30 - 408 - 6.2
40 - 506.2 -4.4
50 - 604.4 - 3.3