Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Open the control box by removing the two securing screws and lower the access door
Draw the control board forward to allow access to the boiler control switch
Disconnect leads from boiler control switch
Open the control box by removing the two securing screws and lower the access door
Draw the control board forward to allow access to the boiler control switch
Disconnect leads from boiler control switch
Measurement:
Test 1:
With switch in O position check for resistance between centre terminal
and R/H terminal (as viewed from front).
Resistance should be open circuit.
Move switch into H position.
Resistance should be short circuit.
Test 2:
With switch in O position check for resistance between centre terminal
and L/H terminal (as viewed from front).
Resistance should be open circuit.
Move switch into L position.
Resistance should be short circuit.
With switch in O position check for resistance between centre terminal
and R/H terminal (as viewed from front).
Resistance should be open circuit.
Move switch into H position.
Resistance should be short circuit.
Test 2:
With switch in O position check for resistance between centre terminal
and L/H terminal (as viewed from front).
Resistance should be open circuit.
Move switch into L position.
Resistance should be short circuit.
Isolate as per TOPs P4
and refit as necessary