Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector CN2 from the main PCB
Remove the AC fan connector (3 wire) from the fan

Measurement:

Test 1:
Measure the resistance between pin 5 (brown) on CN2 and the brown on the fan connector.
Expect continuity.
Test 2:
Measure the resistance between pin 4 (blue) on CN2 and the blue on the fan connector.
Expect continuity.
Test 3:
Measure the resistance between earth on the fan connector and the boiler chassis.
Also inspect the condition of the harness visually checking for any damaged insulation.