Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the injector from the fan venturi
(behind fan, 1 clip then twist to remove)

Observation:

Visually inspect the injector, it should be undamaged and clear of any restrictions or blockages.
Is the injector in good condition?
Note:
If the injector is in good condition do not refit it at this point.