Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the burner door assembly

Observation:

Inspect the primary heat exchanger flueways for any restrictions or blockages.
Notes:
1. Before the combustion chamber door is refitted and the appliance left in operation the burner door gasket MUST be replaced and new nyloc nuts used.
2. If the flueways are satisfactory do not reassemble the appliance until a diagnosis has been reached.