Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect leads from the APS
Connect the air pressure switch charge test kit to P1 (+ve) on the APS
Note the operating pressure which is indicated on APS

Measurement:

Increase the pressure via the syringe to that indicated on the APS.
Measure the resistance between terminals 1 and 3.
Expect continuity / zero resistance.
Note.
1. 100 Pascals = 1mbar / 50 Pascals = 0.5mb.
2. If continuity is not obtained, it is permissible to increase the pressure by 50%