Why wont the fan or blower on my furnace or AC shut off?

Is your furnace or AC fan running continuously?

First, try this: Make sure your thermostat fan setting is on AUTO not ON. Then, turn the thermostat to a temperature that’s lower than the current room temperature (when in heat mode) or higher than the current room temperature (when in cooling mode) and wait 3 to 5 minutes.


If your furnace or AC fan is continusly running, it could be one of these 2 problems:

  • The fan limit switch is faulty or tripped and needs to be replaced
  • There’s a problem in your thermostat wiring

Follow these steps to check if you fan limit switch is set correctly. Otherwise, call Custom Comfort and schedule and appointment.

  • Step 1: Locate your fan limit switch
    • Your fan limit switch is located inside the furnace panel cover, usually near the upper right corner.
    • The fan limit switch is responsible for turning the blower on and off at the appropriate times. So, when the switch is working correctly, your furnace fan should turn on several minutes after thermostat calls for heat and turn off several minutes after the heat cycle ends.
    • But if the furnace fan runs continuously, regardless of whether the furnace is heating or not, your furnace fan limit switch is either set incorrectly to Manual or it is faulty and needs to be repaired or replaced.
  • Step 2: Make sure the white button is NOT pressed in
    • Your fan limit switch will have a small white button on the lower left corner. That button tells the switch whether to operate manually (run the fan all the time) or automatically (run the fan only during heating and cooling cycles).
    • If the button is pressed in, the switch is set to Manual and is forcing the fan to run continuously. You’ll need to reset it to AUTO to allow for proper fan operation.
  • Step 3: Call a professional if the fan is still running
    • If your fan limit switch is set to AUTO but the fan continues to run without ever stopping, you’ll need to call a professional.
    • Two problems that could cause your fan to run without stopping include a bad fan limit switch that needs to be replaced or bad and/or incorrect thermostat wiring.
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