Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH/HW Switch in HW position
Ensure that the boiler is cold and ready for Pre-heat
Boiler On/Off Switch ON
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With the fan running, measure mains voltage across the air pressure switch:
- yellow wire (NO) on the plug-on connector on PCB 26; and
- the Neutral (blue) on the mains supply connector block.