Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the 12 way plug from the front edge of the PCB
Measurement:
Measure the resistance across the two orange wires to the air pressure switch.
RESISTANCE AIR PRESSURE SWITCH AND HARNESS (NO AIR)
Isolate as per TOPs P4
and refit as necessary