Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Ensure the lead is left connected to the gas valve and the PCB
Remove plastic cover from the gas valve lead
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the blue and brown connections on the PCB within the gas valve lead assembly.
Expect mains voltage.
Note:
It is vital that you get a good contact across the brown and blue connections with your probes otherwise inaccurate readings will be given.