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My furnace comes on just fine. It runs for a little while and will raise the temp a degree or two then it shuts off but the blower still blows out cold air. The flame sensor and air filter were replaced, pipes checked for condensation, chimney checked for blockages, thermostat replaced. If I turn off the furnace and turn it right back on it starts all over.

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    Most furnaces have an LED (sometimes behind a little window) that indicates fault codes. It will do a certain combination of short and long flashes, and there should be a label explaining what the codes mean. Can you see if yours is showing any error codes? Also, we would need to know the brand and model of your furnace to help more. Nov 2, 2021 at 13:19
  • Manufacturer/Model #? Nov 2, 2021 at 13:26
  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
    – Community Bot
    Nov 2, 2021 at 13:33
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    It's normal for the blower to continue for a while after the flame is shut off. And of course the air being blown will become increasingly "cold" while that is happening.
    – kreemoweet
    Dec 28, 2022 at 6:25
  • Can you elaborate on how long the fan runs after the burner shuts off? Does it blow for a minute or two before shutting off, or does it blow continually forever until you manually cut power to the furnace?
    – PaSTE
    Oct 12, 2023 at 13:25

2 Answers 2

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Had a similar problem with an old Lennox furnace, both LEDS showed OK. Turned out to be the controller board.

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  • Awe, anything but that. If it isn't the flame sensor (usually) or a pressure sensor, or the high limit switch.... then it's the board... boooo +1.
    – Mazura
    Jan 28, 2023 at 19:39
  • Yes, it took 3 visits from servicefolks.... they changed various sensors and tried to claim it was the thermostat, but in the end a new controller board fixed it.
    – QTX
    Feb 21, 2023 at 20:52
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I will take a SWAG as I do not have a clue as to what furnace or model you have. I have seen this many times years back when the bonnet sensor failed. The ones I am familiar with have two settings, the lower one turns on the blower and the higher one is the safety. If yours has a temperature scale that you can adjust the lower should be maybe 140F and the upper about 180F. You can also get similar results with the blower running to slow or plugged filters. Remove the filter and see if it will run, is so you found your problem.

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