Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove flue thermistor connector
Remove the thermal fuse connector
Remove connector CN11 from the main PCB

Measurement:

Test 1:
Measure the resistance between pin 2 (black) on CN11 and the Black on the flue thermistor connector.
Test 2:
Measure the resistance between pin 1 (black) on CN11 and the Black on the thermal fuse connector.
Test 3:
Measure the resistance between the White on the flue thermistor connector and the White on the thermal fuse connector.
Expect continuity on each test.
Also visually inspect the condition of the harness.