Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads from the air pressure switch noting their positions

Measurement:

Note the operating pressure indicated on the pressure switch. Connect the air pressure switch charge test kit to the P1 connection on the air pressure switch.
Increase the pressure via the syringe to that indicated on the pressure switch.
Continuity / zero resistance should then be measured between common and normally open terminals on the air pressure switch.
Notes:
If continuity / zero resistance is not obtained, it is permissible to increase the pressure by 50%.
100 Pascals = 1mbar / 50 Pascals = 0.5mbar