Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove connector plug R from PCB
Remove connector plug R from PCB
Measurement:
Measure resistance across connection 67 (brown wire, air pressure switch connection C) and connection 68 (black wire air pressure switch connection NC) at the disconnected plug.
Note: Measurement to be taken at the disconnected plug not PCB.
Note: Measurement to be taken at the disconnected plug not PCB.
Isolate as per TOPs P4
and refit as necessary