I went on a beckett burner lock out recently , after a reset and refire the guy says wait... and turns on a box fan which is blowing up and out of a 1/4 open basement window.....
the small hole in the boilers exhaust where you stick a probe for combustion test is profoundly fuming smoke
I say hey, how long you running it like this, as it shuts off and locks out again, hey says a week or so!!
i say you got co detectors? he says yes!! they have been going off upstairs so he pulls thier batteries!!
i disassemble the stack and it was completly plugged.....every turbulator packed.. he said they cleaned it ....5 years ago!!!
sometimes its worth having a good inspection done... they are potential health hazzard