Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the 4 wire plug from the Fan

Measurement:

Measure for continuity (zero resistance) between the 4 wires,
in turn, at connector X23 on the PCB (pins 3 to 6)
to the corresponding colour at the disconnected plug.
Inspect the harness and connectors for good overall condition.