Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Gain access to air pressure switch and disconnect leads
Measurement:
Measure for resistance across terminals 1 & 2 (N/C) of the air pressure switch.
Expected resistance continuity (zero resistance).
Is resistance measured as expected?
Expected resistance continuity (zero resistance).
Is resistance measured as expected?
Isolate as per TOPs P4
and refit as necessary