Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the black and brown leads from the air
pressure switch, making a note of their connections

Measurement:

Measure the resistance between terminal marked N/C (brown lead) and terminal marked COM (black lead) on the air pressure switch.
Expected resistance continuity (zero resistance).