Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove plastic cover from the ignition control PCB attached to the gas valve

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.