why does my high eff gas furnace exhaust run constantly ( when furnace isn't on ) the furnace turns on and off a few times before running then the blower comes on. now the exhaust wont stop without cutting power to the furnace

  • 1
    Does the furnace cycle normally (other than the draft motor staying on)?
    – Tester101
    Apr 22, 2015 at 0:36
  • On a York roof-top unit I've worked on in the past, the draft inducer runs continuously while in lock-out. This is by design and their equivalent of an error code I guess. One of your limit switches is bad?
    – Mazura
    Apr 22, 2015 at 5:36

2 Answers 2


There might be an error code on your electronic control board. It's located in the lower half of the furnace. Sometimes there's a little plastic window that you can look through or you might have to take the cover off.

There'll be an LED on the board that will blink in a special sequence and give you a code number. There should be a label inside the cover or inside the wall of the furnace that tells you how to decipher the code.


Your burners are cycling on and off it is the induced draft that is running on until killing poer if I understand you correctly. If the flue or blower limit were tripped they will not automatically reset themselves or by killing power to the unit then restoring it, nor will flame roll out safeties they must be manually reset and have a little red button to push before they reset. The high limit in the heat exchanger if tripped will initiate lock out and will auto reset after time or killing power as described however the indoor blower will run continuously ass described by another expert and his York unit,this is by design and for safety.To get the ignition module to reset still requires killing power, this is so we do not miss a cracked heat exchanger or similar dangerous condition. The indoor blower motor is controlled by a circuit board or relay with a time or temperature or delay or time on /temperature off delay for the low speed fan in heat mode. If the temperature is being maintained this points to a bad board, to be sure bypass the thermostat by removing the red wire from the thermostats sub base and jumping it to the white terminal, this simulates a heat call and then you can see if it the heat mode operates normally by jumping red to white at the thermostat or at the furnace. A diagnostic light on the board should be flashing and a legend on the blower door tells you what the flash code represents.

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