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 the connections
Remove the air pressure switch from the boiler

Measurement:

Note the operating pressure indicated on the pressure switch.
Connect the air pressure switch charge test kit to 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:
If continuity is not obtained, it is permissible to increase the pressure by 50%.
100 Pascals = 1mbar / 50 Pascals = 0.5mbar