Does anyone know what to do next?

Potterton puma 100 e that goes to lockout as it wont fire with the case on. remove case and burner inspection cover and it fires. I have a van load of spares so have changed the gas valve, air pressure switch, spark electrodes and full sequence and modulation boards, i have checked the vertical flue,fins on exchanger, fan ok, tubes all ok, im lost as what to do next-other than change it!