Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use insulation mat as necessary)
Selector Switch in winter position
Room stat calling for heat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage on J2 terminal 4.