Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the connector from the flue sensor
at the top of the heat exchanger

Measurement:

Measure the resistance across the two pins of the sensor.

The resistance depends on temperature and should
conform to the table below:
Temperature °CResistance in kOhms
20 - 3012.0 - 8.2
30 - 408.5 - 5.5
40 - 505.8- 3.8
50 - 604.0 - 2.7
60 - 702.9 - 2.0
70 - 802.1 - 1.4