Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Disconnect the Air Pressure Tube from the Air Pressure Switch.
Measure the resistance across the Red wire (connection marked L2) and the Black wire (connection marked L3) on the connector at the PCB.
Measure the resistance across the Red wire (connection marked L2) and the Black wire (connection marked L3) on the connector at the PCB.
Isolate as per TOPs P4
and refit as necessary