Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Measurement:
Remove the Black spaded wire marked NC and the white spaded wire marked C on the Air pressure switch and measure the resistance across the NC and C terminals on the Air pressure switch.
Reconnect the two wires after the test.
Reconnect the two wires after the test.
Isolate as per TOPs P4
and refit as necessary