| The original poster made the assumption, by testing, that the parts in question were okay. When in reality, one does not know that until one can witness that they STAY okay when the flame shuts off.
If say a pressure switch is on the cusp of barely working, let's say, and then you test it while it is barely working, it will read good. But what if when the flame shut off, that it no longer is okay, say for reason that just a few extra dribbles of condensate, backed up into the secvondary heat exchanger, now weakens the drafting of the furnace to the point the pressure switch opens up. That is why you have to test immediately right when there is failure.
And/or you can do some routine maintenace stuff like, for one, blow out the condensate line both ways, and it's trap and then refill the trap with water so that CO gases do not come out a trap vent.
Or that the vacuum tube that leads to the poressure switch, is plugged up some at it's source. Not all the way -just SOME, so the switch is always on the cusp. Or that you have SOME ice outside at the exhaust or intake vents. Or that the ignitor is going bad. Not totally bad yet-going bad. Or you have tarnished spade connectors that really need to make better connections. Or instead, you have a jack with those pin connectors that have longitudinal splits in them (by design) where the pins have reduced in diameter, not making totally good contact everytime. (You can spread and enlarge them, for reinstituting better contact once again)) Or the module or board is not grounded as good as it should be.
Also observe what part of the sequence is failing, when it fails. That can tip us off.  |