Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector from ST9 on the PCB

Measurement:

Measure resistance across connections 11 and 12 (orange leads) at the disconnected connector.
Expected resistance closed circuit (zero resistance).
Inspect the flue stat for any signs of corrosion, damage, or moisture.