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 (4wire) from the fan assembly

Measurement:

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

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