Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
On/Off Switch On.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With Fan running measure Mains Voltage across the Black wire connection
7 (Pressure Switch connection NO) and the Blue wire connection 3
(PCB Neutral) on the PCB.
Note:
Test carried out at PCB (control box) not Air Pressure Switch.