Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the leads from the air heater switch

Measurement:

Set the air heater switch to timed.
Measure the resistance across the top (white lead) and the middle (pink lead) terminals. Resistance should be closed circuit.
Set the air heater switch to continuous.
Measure the resistance across the middle (pink lead) and the bottom (grey lead) terminals. Resistance should be closed circuit.
Reconnect leads after test.
Note: The lead colours are shown for information only.