Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove brown and black leads from the heat exchanger frost thermostat (rear most of the two thermostats fitted on the R/H side of the heat exchanger)

Measurement:

Measure resistance across the exposed terminals.
Resistance should be open circuit.
Replace leads after test.
Note:
If conditions are cold (i.e. frosty weather) then thermostat may have switched to short circuit. Thermostat should only be short circuit under cold weather conditions to protect appliance from freezing.