Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the two White wires from the Purge Switch
Disconnect the two White wires from the Purge Switch
Measurement:
Measure the resistance across the Switch.
With the Purge Switch in its PURGE position, an open circuit resistance should be obtained.
When the Switch is set to its NORMAL position, a closed circuit resistance should be obtained.
Replace leads after test.
With the Purge Switch in its PURGE position, an open circuit resistance should be obtained.
When the Switch is set to its NORMAL position, a closed circuit resistance should be obtained.
Replace leads after test.
Isolate as per TOPs P4
and refit as necessary