Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Measurement:
Disconnect the Tubes from the Air Pressure Switch noting their positions.
Disconnect the Brown wire and the Grey wire both connected to the Air Pressure Switch (connection marked C) and the Yellow wire connected to the Air Pressure Switch (connection marked NC).
Measure the Resistance across the C and NC connections on the Air Pressure Switch.
Disconnect the Brown wire and the Grey wire both connected to the Air Pressure Switch (connection marked C) and the Yellow wire connected to the Air Pressure Switch (connection marked NC).
Measure the Resistance across the C and NC connections on the Air Pressure Switch.
Isolate as per TOPs P4
and refit as necessary