Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the 4 wire connector from the fan
Remove connector X401 from the PCB

Measurement:

Measure the resistance of each of the 4 wires (brown, black, blue and green) from the fan connector to the corresponding wire on connector X401.
Also visually check the condition of each of the wires in the harness.
Expect continuity / zero resistance on each of the 4 wires in the harness.
Are the resistances as expected and is the harness in good condition?