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 On.
Resistance between black and blue wires is open circuit, black and orange wires short circuit.
Resistance between black and blue wires is open circuit, black and orange wires short circuit.
Isolate as per TOPs P4
and refit as necessary