Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Remove the plug X2B IGNITER/ FLAME SENSE from the PCB
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow self check to complete and ignition sequence to commence.
Using shrouded probes measure voltage across brown and blue
labelled terminals on the PCB at X2B IGNITER/ FLAME SENSE
on the rear of the lowered control box.
Expect mains voltage.
Note:
Voltage will only be present during the sparking phase
of the ignition sequence.