Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the wires from the air pressure switch noting their locations
Remove the wires from the air pressure switch noting their locations
Measurement:
Test 1:
Measure the resistance at the air pressure switch between the common terminal and the N/C terminal.
Expect closed circuit (continuity / zero resistance).
Test 2:
Measure the resistance at the air pressure switch between the common terminal and the N/O terminal.
Expect open circuit (greater than 1MOhm or O.L).
Are the resistances measured as expected?
Measure the resistance at the air pressure switch between the common terminal and the N/C terminal.
Expect closed circuit (continuity / zero resistance).
Test 2:
Measure the resistance at the air pressure switch between the common terminal and the N/O terminal.
Expect open circuit (greater than 1MOhm or O.L).
Are the resistances measured as expected?
Isolate as per TOPs P4
and refit as necessary