Top causes of a furnace blower that won’t turn off

Is your furnace blower refusing to shut off? A blower that runs non-stop isn’t just a nuisance, it can also spike your energy bills. But don’t worry; we’re here to help you figure out what’s going on and how you can tackle it yourself like a Pro.

The advice in this article might not match your exact appliance model. While we try to provide helpful tips for a wide range of DIY appliance repairs, differences in models and setups can mean variations in troubleshooting steps, error codes, parts, and more. For the most accurate info, we highly recommend checking the manufacturer’s documentation for your specific model or reaching out to us with any questions.

Thermostat issues

One of the first places to check if your furnace blower won’t turn off is the thermostat. The thermostat tells the furnace when to turn on and off, and if it’s malfunctioning, it could cause the blower to keep running.

How to fix it:

1. Make sure your thermostat is set to “auto” and not “on.” The “on” setting will keep the fan running continuously, while “auto” allows it to turn off when the desired temperature is reached.
2. If the thermostat is miscalibrated, it might be telling the furnace to keep running. You can test the thermostat by comparing the actual room temperature to the thermostat setting. If the readings are significantly different, it might need recalibration.
3. If you suspect the thermostat is faulty, use a multimeter to check for electrical continuity. Set your multimeter to the lowest setting for continuity testing.
4. Disconnect the thermostat from the furnace, then attach the probes to the thermostat terminals.
5. If the multimeter doesn’t show continuity when the thermostat is calling for heat, it’s time for a replacement.

Inspect the fan limit switch

The fan limit switch is responsible for controlling when the furnace blower starts and stops. If it’s malfunctioning, it could cause the blower to continue running even after the furnace has reached the desired temperature.

How to fix it:

1. The fan limit switch is usually located near the furnace burner assembly or attached to the furnace plenum. If you’re unsure, consult your furnace’s user manual for its exact location.
2. Inspect the switch for any signs of physical damage or wear. Look for scorch marks or signs that it may have overheated.
3. Set your multimeter to continuity testing and attach the probes to the fan limit switch terminals.
4. Activate the furnace and monitor whether the switch opens and closes correctly. You should hear a click when it switches states.
5. If the switch does not show continuity when it’s supposed to, it may need to be replaced.
6. If the fan limit switch is malfunctioning, turn off the furnace and gas supply for safety.
7. Disconnect the wires from the old switch, remove any screws or bolts securing it, and install the new switch in the same position.
8. Reconnect the wires and secure the switch in place.

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Check the blower relay

The blower relay controls the electrical signal that powers the furnace blower. If the blower relay is stuck or malfunctioning, it may cause the blower to keep running even after the furnace has reached the set temperature.

How to fix it:

1. The blower relay is usually located on the furnace control board. Refer to your furnace’s manual if you’re not sure where to find it.
2. Visually check the relay for any signs of damage, such as burns, cracks, or loose connections. A faulty relay can often appear burned out or corroded.
3. Set your multimeter to continuity or resistance and touch the probes to the relay terminals.
4. If the multimeter shows continuity even when the furnace is off, or if the relay doesn’t close when the furnace calls for heat, it is likely faulty.
5. If the relay is stuck or not functioning correctly, you will need to replace it. Turn off the furnace and disconnect the power.
6. Remove any screws or fasteners securing the relay to the furnace control board.
7. Disconnect the wires and replace the old relay with the new one, ensuring the wires are reattached in the same configuration.

Inspect the blower motor

The blower motor is responsible for circulating air throughout the furnace system. If it’s malfunctioning, it could cause the blower to run continuously. Overheating or electrical issues with the motor can also prevent it from turning off as needed.

How to fix it:

1. The blower motor is typically located inside the furnace near the air handler or blower assembly. If you’re unsure where it is, check the furnace manual for the exact location.
2. Inspect the motor for any visible damage such as rust, wear, or overheating. You should also look for any signs of debris or dust buildup around the motor or blower wheel.
3. Turn on the furnace and listen for any unusual sounds, such as grinding, squeaking, or humming. These sounds may indicate that the motor bearings are worn or that the motor is struggling to turn.
4. Set your multimeter to measure continuity or resistance and test the motor’s electrical connections.
5. If the multimeter doesn’t show continuity, the motor may be faulty and needs to be replaced.
6. If your blower motor is a type that can be lubricated, apply lubricant to the motor bearings to reduce friction and improve operation.
7. If the motor is faulty or shows signs of overheating, you will need to replace it. Turn off the power, disconnect any electrical wiring, and remove any screws or bolts holding the motor in place.
8. Install the new motor in the same position, reconnect the wiring, and secure it with the appropriate fasteners.

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Inspect the control board

The control board in your furnace controls various functions, including the blower motor. If there’s a problem with the control board, it could send incorrect signals, causing the blower to run continuously. A malfunctioning control board could be the reason your blower won’t turn off.

How to fix it:

1. The control board is typically located in the furnace’s main panel, near the burner area. It’s often a small circuit board with wires attached to it.
2. If you’re unsure about the location, check your furnace manual for specific guidance.
3. Look for visible signs of damage, such as burned components, scorch marks, or corrosion. Any of these issues could indicate a problem with the control board.
4. Check if any wires are loose or disconnected, as this could affect the signals sent to the blower motor.
5. Set your multimeter to test for continuity or resistance.
6. Disconnect the power to the furnace and remove the control board cover to expose the board.
7. Test the individual components on the board for continuity. If any component is faulty or doesn’t show continuity where it should, this part of the control board may need to be replaced.
8. If the control board is faulty, turn off the furnace and disconnect the power supply.
9. Disconnect the wires from the old board and unscrew it from its housing.
10. Install the new control board in the same position, reconnecting the wiring according to the furnace’s schematic.
11. Secure the new control board in place with screws or mounting brackets.

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Wrapping it up

We hope this guide helps you get to the bottom of why your furnace blower won’t turn off. 

For all your furnace parts and more detailed repair guides, visit us at AppliancePartsPros.com. We’re here to help you every step of the way. Stay connected for more tips and updates: Follow us on Facebook and Twitter. If you have questions or need more assistance, don’t hesitate to contact us. We’re always happy to help!