Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the leads from the water heater switch
Remove the leads from the water heater switch
Measurement:
Set the water heater switch to timed.
Measure the resistance across the top were the (white lead) and the middle (violet lead) were position on the terminals. Resistance should be closed circuit.
Set the water heater switch to continuous.
Measure the resistance across the middle were the (violet lead) and the bottom (grey lead) were position on the terminals. Resistance should be closed circuit.
Reconnect leads after test.
Note: The lead colours are shown for information only.
Measure the resistance across the top were the (white lead) and the middle (violet lead) were position on the terminals. Resistance should be closed circuit.
Set the water heater switch to continuous.
Measure the resistance across the middle were the (violet lead) and the bottom (grey lead) were position on the terminals. Resistance should be closed circuit.
Reconnect leads after test.
Note: The lead colours are shown for information only.
Isolate as per TOPs P4
and refit as necessary