Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect flue sensor inline connector
Measurement:
Measure the resistance across the pins of the inline connector to the flue sensor.
RESISTANCE FLUE TEMPERATURE SENSOR
Resistance varies with temperature and should conform approximately to the graph below:
Isolate as per TOPs P4
and refit as necessary