My oil burner at first would not fire off. Changed filters and nozzle. Furnace ran fine for a week or so then tripped off again. Cleaned cad cell and it fired and ran for a day, then tripped off once more. Got it running again but last time burner continued to run after flame extinguised, flooding the fire box with oil. In the process of thorough cleaning of furnace. Do you think cadcell is bad?

I think now would be a good time for you to call for ON Site Service since with a belly full of oil you may not want to light it off for fear of causing damage.