Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect leads to the air pressure switch (note their locations)
Measurement:
Measure for resistance across terminals 1 & 3 of the air pressure switch.
Expected resistance open circuit.
Is the resistance measured as expected?
Expected resistance open circuit.
Is the resistance measured as expected?
Isolate as per TOPs P4
and refit as necessary