Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the leads from the air pressure switch making a note of their positions
Measurement:
Measure the resistance between terminals 1 and 3 on the air pressure switch.
Expected resistance open circuit.
Expected resistance open circuit.
Isolate as per TOPs P4
and refit as necessary