Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove APS from boiler (located top right behind combustion chamber)

Measurement:

Note the operating pressure indicated on the pressure switch.
Connect the air pressure switch charge kit to +ve connection
on the air pressure switch (labelled P1).
Increase the pressure via the syringe to that
indicated on the pressure switch.
Measure between the two terminals of the air pressure switch.
Expect continuity / zero resistance.
Note:
If continuity is not obtained when the specified pressure
is applied it is permissible to increase the pressure by 50%
100 Pascals = 1mbar / 50 Pascals = 0.5mbar