Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect low voltage plug from the fan

Measurement:

Check for continuity (zero resistance) between the ends of the red, white, black and yellow wires running from the fan to pins 4, 5, 6 and 7 at X4 on the PCB.
Inspect the harness and connectors for good overall condition.