Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove electrical leads from air pressure switch

Measurement:

Measure the resistance across the exposed terminals of the air pressure switch.
Expected resistance continuity (zero resistance).
Note:
If flueway to sump connection is disturbed a new
sump seal must be fitted, refer to Adapt for guidance.