AC Fan Running Constantly (Troubleshooting Guide)

On this page

Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

If your AC fan runs constantly, check the thermostat settings, air filter, and fan switch. A dirty filter or incorrect settings are common causes. If these don't resolve the issue, the problem may be with the fan relay or control board.

Confirm the symptom

AC fan runs nonstop even when cooling is not needed
Fan continues after AC cycle ends
Thermostat is set to 'Auto' but fan still runs constantly

Tools and prep

Screwdriver set
Thermometer
Vacuum or new air filter
Flashlight
Turn off power to AC unit at circuit breaker before working

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Auto' for both cooling and fan modes.
Observe: If set to 'Fan On', the fan will run continuously.
Means: Incorrect thermostat setting causing constant fan operation.
Next: Change to 'Auto' and monitor fan behavior.
Inspect Air Filter
Do: Remove and examine the air filter for dirt or blockage.
Observe: A dirty filter restricts airflow, causing the fan to work harder.
Means: Clogged filter increases system strain and may trigger constant fan use.
Next: Replace or clean the filter if necessary.
Check Fan Switch
Do: Locate the fan switch on the thermostat or air handler and ensure it's in the 'Auto' position.
Observe: If the switch is in 'On', the fan will run constantly.
Means: Manual fan switch override is active.
Next: Switch to 'Auto' and observe results.
Test Fan Relay
Do: Turn off power to the unit. Locate the fan relay in the air handler and inspect for damage or sticking contacts.
Observe: A faulty relay may keep the fan energized.
Means: Relay failure is causing continuous fan operation.
Next: Replace the relay if damaged.
Check Control Board
Do: Inspect the control board for visible damage, such as burn marks or loose connections.
Observe: Malfunctioning board may send incorrect signals to the fan.
Means: Control board issue is causing the fan to run constantly.
Next: Consult a professional for board replacement or repair.
Verify System Pressure
Do: Use a thermometer to check the temperature difference between the evaporator coil and return air.
Observe: A small temperature difference may indicate low refrigerant or other system issues.
Means: System imbalance could cause the fan to run longer to compensate.
Next: Contact a professional for refrigerant checks and system diagnostics.

Common causes

Cause	Likelihood	Difficulty	Part often needed
Thermostat Set to 'Fan On' The thermostat controls fan operation; 'Fan On' setting overrides cooling cycles.	Common	Easy	None
Dirty Air Filter A clogged filter restricts airflow, causing the system to work harder and the fan to run longer.	Common	Easy	Air filter
Stuck Fan Relay A faulty relay may keep the fan energized even when it should stop.	Sometimes	Medium	Fan relay
Faulty Control Board A malfunctioning board can send incorrect signals to the fan motor.	Sometimes	Hard	Control board
Low Refrigerant Low refrigerant causes the system to run longer to cool, keeping the fan active.	Rare	Hard	Refrigerant
Incorrect Fan Switch Position A manual fan switch in the 'On' position forces the fan to run continuously.	Common	Easy	None

Parts matching

Common parts needed for resolving constant AC fan issues include:

Air filter for improved airflow
Fan relay for electrical control
Control board for system coordination

Always ensure the power is off before replacing parts to avoid electrical hazards.

When to call a professional

Fan continues to run after checking thermostat settings and replacing the air filter
Visible damage to the control board or electrical components
Suspected refrigerant leak or system imbalance
Unfamiliarity with electrical components like relays or control boards
Fan relay or control board replacement required

FAQ

Can a dirty air filter cause my AC fan to run constantly?

Yes, a clogged air filter restricts airflow, making the system work harder and potentially causing the fan to run longer than necessary.

Why does my AC fan run even when the thermostat is set to 'Auto'?

This could be due to a stuck fan relay, a faulty control board, or incorrect thermostat settings. Check the thermostat and inspect the relay for issues.

Is it normal for the AC fan to run after the cooling cycle ends?

Brief post-cycle fan operation is normal to circulate conditioned air, but constant running indicates an issue.

How can I tell if the fan relay is faulty?

A faulty relay may show signs of damage, such as burn marks or sticking contacts. If the fan runs continuously despite correct settings, the relay may be the culprit.

Can low refrigerant cause the fan to run constantly?

Yes, low refrigerant can cause the system to run longer to achieve the desired temperature, keeping the fan active. This requires professional attention.