Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X100 from PCB

Measurement:

Check continuity of the air pressure switch harness, pins 1 & 3 of connector X100 to connections at the air pressure switch.
Expected resistance, continuity (zero resistance) on each wire of the harness.