Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove red wires from primary sensor
Ensure primary heat exchanger is below 60° C
Carefully remove sensor.

Measurement:

Measure resistance across the exposed sensor terminals.
The resistance should conform to the table below:
Temperature °CResistance in kOhms
20 - 308.0 - 16.0
30 - 40 6.0 - 10.0
40 - 503.9 - 8.0
50 - 602.5 - 5.5
60 - 701.5 - 3.0

Refit sensor after test