Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Check venturi connection in the flue air restriction ring for blockage.
Note:
It is not advisable to remove the connection unless absolutely necessary. The nut that the connection screws into is not captive and it may be difficult to refit both the nut and the connection to the flue ring.
If the connection must be removed, then care should be taken not to drop the nut or the connector as it may be difficult to retrieve.