Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the fan from the burner mounting assembly
(take care not to damage the cork gasket)
Remove the air / gas mixing nozzle

Observation:

Visually inspect the condition of the air gas mixing nozzle, all the holes should be clear and free from restrictions, the inner and outer surfaces of the nozzle should also be smooth and clean.
Is the nozzle in good condition, clean and unrestricted?