Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove both the tubes from the air pressure switch, noting their colour coded locations.
Measure the resistance across the air pressure switch plug-on connector red wire (C), and the air pressure switch plug-on connector violet wire (NC) at PCB 26.
Refit both tubes to the air pressure switch.
Measure the resistance across the air pressure switch plug-on connector red wire (C), and the air pressure switch plug-on connector violet wire (NC) at PCB 26.
Refit both tubes to the air pressure switch.
Isolate as per TOPs P4
and refit as necessary