Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove plastic cover from the ignition control PCB attached to the gas valve
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 NC on the air pressure switch.
Resistance should be short circuit.
These leads connect to terminals C and NC on the air pressure switch.
Resistance should be short circuit.
Isolate as per TOPs P4
and refit as necessary