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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?

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    Does your furnace provide error codes? Another common fault is the flame sensor, which can often be cleaned. – isherwood Jan 29 '18 at 21:05
  • The furnace is ~25 years old and does not provide error codes, though cleaning the flame sensor certainly wouldn't hurt, I'll give that a try. – Barry Jan 30 '18 at 0:33
  • Merely FYI I replaced my 23 year old furnace's control board with an aftermarket board that does provide error codes. It was an easy swap for about $75. – isherwood Jan 30 '18 at 1:32
  • Thanks for that suggestion, I'll look into that as well. – Barry Jan 31 '18 at 4:20
  • As per my update above, cleaning the flame sensor hasn't resolved the problem, though I have more info with the LED error code. – Barry Feb 1 '18 at 6:39
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Nest is well known to cause short cycling. Go back to your old thermostat.

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    Hello, and welcome to Stack Exchange. I've never heard this; would you please give us some sort of source for this somewhat extreme statement? – Daniel Griscom Nov 29 '18 at 20:08
  • The furnace is not short cycling. It enters into a condition where the blower motor remains on but the furnace won't ignite. It remains in this condition until the furnace is told to turn the heat off (either by turning down the heat, cutting power, etc.) – Barry Dec 17 '18 at 15:26

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