Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the fan running measure the voltage between the yellow wire marked "NO" at the plug for the air pressure switch (RHS, refer to pictures) to a suitable earth point.
Mains voltage should be present.