Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect plug for thermal fuse / flue thermistor

Measurement:

Measure the resistance across the thermistor component of the combined thermistor / fuse.
Thermal fuse / thermistor is factory fitted with the catch at the top but can also
be rotated by 180°, refer to pictures and ensure test is done against correct pins.