Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the fan and inspect the venturi

Observation:

Carefully inspect the venturi, are there any restrictions which could affect the air intake?
Note:
1. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
2. Upon reassembly all disturbed joints should be checked for gas tightness.
3. If the venturi is satisfactory do not refit it at this stage.