Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
DHW Demand
On/Off Switch ON
Gain access to PCB (use insulation mat as necessary)
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the ignition sequence to commence.
Measure the DC voltage at the gas valve connector on the PCB between the Blue and Black wires during the sparking stage of the ignition cycle.
Expected voltage 150 volts DC.