Set Appliance State:

Mains Power Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Remove plastic cover from the PCB

Measurement:

Allow the fan to run and the ignition sequence to commence.
Expect approx. 100V AC
Check for mains voltage at connector M10 on the PCB across the red and yellow leads