Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads and tube from the air pressure switch
(make a note of their connections)
Disconnect leads and tube from the air pressure switch
(make a note of their connections)
Measurement:
Note the operating pressure indicated on the pressure switch. Connect the air pressure switch charge test kit to P1 (+ve connection) on the air pressure switch.
Increase the pressure via the syringe to that indicated on the pressure switch, Continuity should then be measured between common (pin 1) and normally open (pin 3) terminals on the air pressure switch.
Note: If continuity is not obtained, it is permissible to increase the pressure by 50%.
100 Pascals = 1mbar / 50 Pascals = 0.5mbar
Increase the pressure via the syringe to that indicated on the pressure switch, Continuity should then be measured between common (pin 1) and normally open (pin 3) terminals on the air pressure switch.
Note: If continuity is not obtained, it is permissible to increase the pressure by 50%.
100 Pascals = 1mbar / 50 Pascals = 0.5mbar
Isolate as per TOPs P4
and refit as necessary