Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector CN9 from PCB.
Remove both wires from the Flue Stat.

Measurement:

Measure the resistance of each of the 2 wires from the fan stat to connector CN9 (pins 1 and 4)
Expect continuity / zero resistance.
Also visually inspect the condition of the harness.