Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the wires from the air pressure switch noting their locations and make safe
Measurement:
Measure resistance between the common and normally open terminals on the air pressure switch.
Expect open circuit (O.L).
Note:
The appearance of the air pressure switch and terminal locations may differ but the test points remain the same - common to normally open.
Expect open circuit (O.L).
Note:
The appearance of the air pressure switch and terminal locations may differ but the test points remain the same - common to normally open.
Isolate as per TOPs P4
and refit as necessary