Set Appliance State:
Isolate Mains Power to appliance as per TOPs P4
Disconnect plug from the flue thermistor
If access is poor you may wish to remove thermistor from flue collector
Disconnect plug from the flue thermistor
If access is poor you may wish to remove thermistor from flue collector
Measurement:
Measure the resistance between the 2 pins for the flue thermistor.
Resistance should conform approximately to the table below.
Resistance should conform approximately to the table below.

Isolate as per TOPs P4
and refit as necessary