Set Appliance State:

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

Measurement:

Measure Mains Voltage on connector J10 connection 2 (brown L) and 1 (blue N).