Set Appliance State:

Mains Power Off
Set Boiler Control Thermostat to MAX position
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Ensure external control are calling for heat
Use insulation mat as required
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the two terminals with purple wires on connector PL3 at the PCB.
Mains voltage expected.