If your furnace blower won’t turn off, check the thermostat first: the fan setting is probably on “On” instead of “Auto”, which runs the blower all the time. If the fan is on Auto and the blower still runs nonstop, the usual causes are an overheated furnace (the blower keeps running to cool it down), a stuck fan relay on the control board, a short in the thermostat wire, or on older furnaces a fan limit switch set to manual.
Last updated: November 2026. Applies to most gas and electric furnaces with a forced-air blower.
Quick diagnosis
| What you notice | Likely cause | Where to look |
|---|---|---|
| Fan runs, no heat, thermostat fan on “On” | Fan setting | Step 1 |
| Fan runs nonstop after the heat shuts off, burners off | Overheating / high limit tripped | Step 2 |
| Fan keeps running with thermostat fan on Auto and the system off | Stuck relay, wiring short, or control board | Steps 3 to 5 |
| Fan runs constantly, older furnace with a dial on the side | Fan limit switch on manual | Step 6 |
| Fan runs and a status light is blinking | Furnace is reporting a fault | Step 7 |
1. Set the thermostat fan to “Auto”
With the fan set to On, the blower runs all the time, even between heating cycles, and the air between cycles feels cool. Set the fan to Auto and wait a few minutes. If the blower stops after the next heating cycle ends, that was the problem.
Some smart thermostats also have a fan circulation or “minimum fan runtime” setting that runs the blower for a set number of minutes every hour. Check your thermostat’s fan settings.
2. Check for overheating
When a furnace overheats, the high limit switch shuts off the burners and the control board keeps the blower running to cool the heat exchanger. If it keeps overheating, the blower can seem to run constantly. The most common cause is a dirty filter. Replace it, open all supply vents and see if normal cycling returns. See furnace high limit switch.
3. Turn the thermostat to Off and watch
Set the thermostat system mode to Off and the fan to Auto. If the blower keeps running:
- Turn off power at the furnace for 30 seconds and back on. If the blower stays off until the next heat call, the control board may have been in a fault or lockout.
- If the blower starts again right away with the thermostat off, the fan is being powered without a call from the thermostat. Continue to steps 4 and 5.
4. Check the thermostat wiring
With power off at the furnace, remove the thermostat from the wall. If the blower stops when the thermostat is disconnected and power is restored, the thermostat or its wiring is the problem. A damaged thermostat cable where the R and G wires touch can run the fan constantly. See thermostat wiring colors.
If the blower still runs with the thermostat wires disconnected, the problem is at the furnace.
5. Stuck fan relay or failing control board
The furnace control board uses a relay to switch the blower on and off. Relay contacts can weld shut and keep the blower running no matter what the thermostat says. This needs a technician to confirm and usually means replacing the control board.
6. Older furnaces: the fan limit switch
Many older furnaces use a fan limit control: a round dial with a small button or tab on the front, mounted on the furnace cabinet. If the button is pulled out or set to Manual, the blower runs continuously. Push it in to Auto. If the dial’s settings have been changed, have a technician reset them.
7. Look at the status light
Most modern furnaces have an LED on the control board. A blinking code while the blower runs often points to a limit or flame problem. Write the code down before you reset power, then look it up: furnace blinking light codes, or the guides for Carrier, Goodman, Trane and Lennox.
A flame rollout code or a blower running with the furnace locked out can signal a safety problem. Don’t keep resetting it; call a technician.
Is it bad to leave the furnace fan running?
Running the fan constantly on purpose isn’t harmful, and some people do it to even out temperatures or improve filtration. But it uses more electricity, especially with older blower motors, and can make the air feel cooler between heating cycles. If the blower runs and you didn’t ask it to, find out why: overheating and stuck relays are problems that won’t fix themselves.
When to call an HVAC technician
Call a technician if the blower runs with the thermostat disconnected, the furnace shows a limit or rollout code, overheating continues with a clean filter, or you smell burning. For a furnace that runs the blower but never heats, also see furnace blowing cold air.
Frequently asked questions
Why does my furnace fan keep running after the heat shuts off?
It’s normal for the blower to run for a minute or two after the burners shut off, to use up the remaining heat. If it runs much longer, check the thermostat fan setting and the filter.
Can a thermostat cause the blower to run constantly?
Yes. A fan setting of “On”, a circulation setting, a faulty thermostat or a shorted wire can all keep the blower running.
How much does it cost to fix a furnace fan that won’t turn off?
A thermostat fix can be free. A new control board usually costs about $300 to $700 installed, depending on the furnace.
Disclaimer: this guide is for general information only. Furnaces involve gas, combustion and high voltage. Turn off power before opening any panel. When in doubt, contact a licensed HVAC professional.