Set Appliance State:
Gas Off
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Measurement:
Remove the Brown wire and Black wire from the Heat Exchanger Frost Thermostat and measure the resistance across the Thermostat.
Note: If conditions are cold (i.e. Frosty Weather) then stat may have switched to short circuit, stat should only be short circuit under cold conditions.
Reconnect wires after test.
Note: If conditions are cold (i.e. Frosty Weather) then stat may have switched to short circuit, stat should only be short circuit under cold conditions.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary