Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
No Demands
Ensure boiler is not in lockout condition
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between connector X1 (flame rectification lead) and the neutral at the incoming mains terminal strip.
Expect approximately 160 vac.