Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads from the air pressure switch noting the connections.
Disconnect leads from the air pressure switch noting the connections.
Measurement:
Note the operating pressure indicated on the pressure switch. Connect the air pressure switch charge test kit to the -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 and normally open terminals on the air pressure switch.
Note:
Due to the type of air pressure switch negative pressure must be used. To give negative pressure start with the syringe fully pushed in and slowly retract.
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 and normally open terminals on the air pressure switch.
Note:
Due to the type of air pressure switch negative pressure must be used. To give negative pressure start with the syringe fully pushed in and slowly retract.
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