Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
CH Switch in CH Continuous position
CH Switch in CH Continuous position
Measurement:
Disconnect the Main PCB Connector marked PL6 from the PCB.
Measure the resistance across the Brown and Yellow wires on the CH Selector Switch.
Replace connector after test.
Measure the resistance across the Brown and Yellow wires on the CH Selector Switch.
Replace connector after test.
Isolate as per TOPs P4
and refit as necessary