Set Appliance State:

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

Measurement:

Measure the voltage between the top pin (blue wire)
and bottom pin (red wire) on connector CN4
Expected voltage should be greater than 7 Vac.