Set Appliance State:
Gas Off
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect green wires from the Air Pressure Switch
Measurement:
Measure resistance across the terminals marked COM and NO on the Air Pressure Switch.
Reconnect wires after test.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary