Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
CH Demand
Gas Off
Selector Switch ON
Remove the two Grey wires from the air pressure switch and place in safe position, clip multimeter leads to the switch terminals.
Selector Switch ON
Remove the two Grey wires from the air pressure switch and place in safe position, clip multimeter leads to the switch terminals.
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the resistance with the Fan running.
Expected measurement short circuit.
Expected measurement short circuit.
Isolate as per TOPs P4
and refit as necessary