Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the front panel from the air/gas mixer assembly

Observation:

1. Confirm the compensation pipe orifice is not restricted and/or blocked.
2. Remove the mixer and the differential nozzles from the chamber.
Visually inspect the mixer and the differential nozzles for restrictions/blockages and general good condition.
Note.
Refer to adapt regarding any gasket replacement requirements.