Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector CN7 from the PCB
Disconnect AC connector (3 wire) from the fan

Measurement:

Test 1:
Measure the resistance between pin 1 (Brown)
on the fan connector and pin 49 (Brown) on connector CN7.
Test 2:
Measure the resistance between pin 3 (Blue)
on the fan connector and pin 48 (Blue) on connector CN7.
Test 3:
Measure the resistance between pin 2 (Earth)
on the fan connector and the boiler chassis.
Expect continuity/zero resistance on each of the above tests.
Also visually inspect the condition of the harness.