Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect plug from air pressure sensor

Measurement:

Check for continuity (zero resistance) between pins 7 (yellow), 8 (green) and 9 (purple) at the fan board to the wire of corresponding colour at the APS connector.
Inspect the harness and plugs for good overall condition.