Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove gas injector from the fan venturi

Observation:

Inspect the injector for restrictions, blockages or damage, ensure that it is completely clean. Inside of the injector should be clean brass & not black (see image).
NOTE:
Be sure that all gaskets are intact and in good condition and all joints are tested for gas tightness when refitting.