Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

1. Measure voltage between Neutral and the black wire on the Summer/Winter Switch.
Mains should be present.
2. Press Gas Valve Switch.
Mains should be absent