Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect wires from APS
Note: APS fitted top right on 40kW appliances

Measurement:

Measure the resistance between the exposed spades of the air pressure switch (marked 1 and 2 on the APS).
Resistance should be short circuit (zero resistance).
Note:
If no APS is fitted choose "resistance short circuit".