Set Appliance State:
Gas Off
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
CH Demand
Remove and make safe both wires from the Air Pressure Switch
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow the fan to reach full speed and read the resistance of the APS.
The resistance should be a short circuit.
The resistance should be a short circuit.
Isolate as per TOPs P4
and refit as necessary