Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the leads from the air pressure switch (make a note of the connections)
Measurement:
Measure the resistance between the common and the N/O terminals on the air pressure switch.
Expected reading is open circuit.
Expected reading is open circuit.
Isolate as per TOPs P4
and refit as necessary