Furnace info
Type: York Stellar Plus (HE down-flow condensing)
Model number: P2UDD12N09501C
Problem
My furnace is intermittently failing, i.e. it stops producing heat. When it fails, the induction blower motor is running, but nothing else is happening. Turning the heat off (either via my Nest thermostat or via power breaker) and back on resolves the problem in that the induction motor will start running again and the furnace will start producing heat again as well. There don't appear to be any discernible factors that determine how long it will take for the problem to recur. It will recur anywhere from several days to only a few hours.
Question
The pressure switch is obviously reverting to an open state at some point, thus disabling the heat. I've confirmed there is no obstruction of the air intake. My question is how do I determine if the pressure switch is faulty or if there are other conditions that are causing the pressure switch to open? It seems any such conditions automatically correct themselves when power is reset, leading me to believe it's just a faulty switch.
Update
I cleaned the flame sensor as suggested, however, the problem persists. (If anything it's now occurring more frequently.)
After further investigation I found there is an LED indicator on the control board (Honeywell S8910U) that I hadn't previously seen (it's behind a panel that needs to be in place during operation), and when the problem occurs the LED flashes on and off (one flash), indicating the control module is in ignition lockout mode.
I don't know what is causing this, and I'm not really any closer to knowing if this is a faulty pressure switch, a bad flame sensor, or something else. Any suggestions for how to isolate the problem so I can replace the appropriate part?
Best Answer
Nest is well known to cause short cycling. Go back to your old thermostat.