Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Lower the controls panel assembly
Measurement:
Disconnect the white spaded wire from terminal marked (NC) and the brown spaded wire from terminal marked (C) on the air pressure switch and measure the resistance across the air pressure switch terminals.
Isolate as per TOPs P4
and refit as necessary