Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Selector switch in CH & HW position.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Remove cover from Main Gas Valve Solenoid. Measure Mains voltage across Brown and Blue wires.