Set Appliance State:

Mains Power Off
Gas Off
Isolate Mains Power to appliance
as per TOP's P4
Set Multimeter to Resistance Set meter to measure resistance
Remove plastic cover from the PCB
Disconnect the connector at M3 from the PCB

Measurement:

Measure resistance across the unplugged connector M3 (blue and white wires).
These leads connect to terminals (C and NO) on the air pressure switch.
Resistance should be open circuit.