Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the burner from the top of the heat exchanger
(You will need to remove the fan and gas section first)

Observation:

Check that the baffles are correctly positioned inside the heat exchanger, remove and inspect them to ensure that they are not corroded or damaged.
Carefully replace in the correct position.
Note:
This component requires the burner to be removed therefore the gasket between the burner and the heat exchanger must be changed.
Gasket - H42-719