Set Appliance State:

Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Boiler On/Off Switch ON
CH Switch OFF
DHW Switch in INSTANT position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the black wire and the brown wire at the gas valve solenoid.