Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove burner door assembly c/w fan and gas valve

Observation:

Inspect the air / gas mixer (venturi) and fan assembly for
blockages / restrictions and good overall condition.
Note.
1. Do NOT reassemble the appliance if the visual inspections are satisfactory.
2. Always examine any seals or gaskets, replacing where necessary.