Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Selector Switch ON
Gas Control Knob in Main Burner position
Disconnect connector CN4

Measurement:

Selector Switch OFF.
Check for open circuit resistance across Selector Switch (Brown wires on connector block CN4).
Selector Switch ON.
Check for short circuit resistance across Selector Switch (Brown wires on connector block CN4).