Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X20 from the PCB
Remove the DC fan connector from the fan assembly

Measurement:

Measure the resistance of each of the wires in the fan harness as listed in the table below.
Pin on connector (X20)Fan DC Connector
3 (Black)Black
4 (Grey) Grey
16 (Blue)Blue
17 (Red)Red

Expect continuity / zero resistance on each of the wires.
Also visually inspect the condition of the harness.
Note:
Each pin on the PCB connector is numbered.