Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4

Measurement:

Measure resistance across connection 5 (black wire) and connection 7 (brown wire) on the molex connector on the ignition PCB.
These leads connect to terminals C and N/C on the air pressure switch.
Resistance should be short circuit.