Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the air / gas mixing nozzle
(behind fan, 1 clip then twist to remove)

Observation:

Visually inspect the condition of the air gas mixing nozzle, it 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?