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 from X2B IGNITER / FLAME SENSE
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Using shrouded probes measure the voltage across clear and blue labelled terminals on the PCB at X2B IGNITER / FLAME SENSE on the rear of the lowered control box.
Expect 130 V AC.