Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove air intake from appliance
Disconnect wires from flue sensor
Disconnect wires from flue sensor
Measurement:
Inspect thermostat for signs of corrosion and/or water ingress.
Measure the resistance across the flue sensor.
Expected resistance short circuit (zero resistance).
Measure the resistance across the flue sensor.
Expected resistance short circuit (zero resistance).
Isolate as per TOPs P4
and refit as necessary