Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW Demand
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between connector X1 and the neutral at the incoming mains terminal strip as the flame ignites.
Expect the voltage to drop from 160v down to approximately 130v when the flame lights.
Note:
It may be necessary to reset the appliance during this test.
Click Here for Guidance on Resetting the Appliance