Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in mid (0) position.
Remove Change Over Valve Microswitch cover.
Remove plug from base of Microswitch housing.
Selector Switch in mid (0) position.
Remove Change Over Valve Microswitch cover.
Remove plug from base of Microswitch housing.
Measurement:
HW Tap OFF.
Resistance between black and blue wires is short circuit, black and orange wires open circuit.
Resistance between black and blue wires is short circuit, black and orange wires open circuit.
Isolate as per TOPs P4
and refit as necessary