AC Fan Running Constantly – Causes, Fixes, and When to Call a Professional

If your AC fan is running constantly, even after reaching the thermostat setting, you may be wasting energy and shortening your system’s lifespan. This issue can stem from thermostat settings, airflow restrictions, or electrical faults like a stuck relay or contactor. Below is a practical troubleshooting guide with clear steps and when professional help is needed.

Quick 60-Second Troubleshooting

Common Reasons AC Fan Runs Constantly

1. Thermostat Fan Set to “On”

When the thermostat fan is set to On, the fan runs continuously regardless of cooling cycles. Switch to Auto so the fan only operates when actively cooling.

Solution: Change thermostat setting from On to Auto. This simple adjustment can reduce energy consumption by 15-20%.

2. Poor Airflow or Dirty Filter

Restricted airflow makes the system work longer to reach target temperature. Replace air filters every 1-3 months to maintain proper airflow and system efficiency.

3. Stuck Fan Relay or Control Board

A failed fan relay or control board can keep the blower powered even when cooling stops. This requires electrical testing and part replacement by a licensed technician.

4. Stuck Contactor or Outdoor Capacitor Fault

If the outdoor fan never stops, the contactor could be welded closed or the capacitor may have failed. These electrical components should be checked by a professional to prevent compressor damage.

5. Wiring or Thermostat Issues

Miswired or shorted thermostat connections can energize the fan continuously. After thermostat replacement, double-check wiring diagrams or call a professional for verification.

6. Coil or Refrigerant Problems

When your system can’t reach setpoint due to dirty coils, refrigerant leaks, or frozen evaporator, it may run endlessly. Professional diagnostics can identify refrigerant issues and airflow blockages.

What You Can Try Yourself

1. Set thermostat Fan to Auto
2. Replace dirty air filters
3. Ensure vents and returns are open and unblocked
4. Turn off any “Circulate” or “Humidity” fan settings in smart thermostats
5. Verify outdoor condenser area is clear of leaves and debris

Warning: If fan continues running after these checks, electrical components may be stuck or failed. Avoid DIY electrical repairs—call a licensed HVAC technician.

When to Call a Professional

If the fan still runs constantly after verifying thermostat and filter, you may have:

• A stuck relay, contactor, or control board failure
• A wiring short between R and G terminals
• Poor cooling due to low refrigerant or airflow issues
• Coil icing or mechanical wear requiring professional repair

A licensed HVAC technician can inspect electrical components, check refrigerant charge, test air temperature split, and perform complete system diagnostics.

Prevent Continuous Fan Operation

• Replace air filters every 1-3 months
• Schedule annual tune-ups for coil cleaning and electrical checks
• Educate family members on thermostat modes (Auto vs. On)
• Seal leaky ducts to reduce system runtime and improve efficiency

Related HVAC Problems and Services

If you’re experiencing other AC issues, GoTech Air Conditioning and Heating provides comprehensive cooling services:

• AC Repair – Fast diagnostics and expert repairs for all cooling problems
• Thermostat Installation – Smart thermostat upgrades and calibration
• Preventive Maintenance – Annual tune-ups prevent 95% of AC breakdowns
• Common HVAC Problems – Troubleshooting guides for homeowners

Frequently Asked Questions

Why does my AC run constantly but doesn’t cool well?

Common causes include airflow restrictions (dirty filter or coils), excessive heat gain, low refrigerant, or duct leaks preventing the system from reaching setpoint. Start by replacing the filter and opening vents. If the problem persists, schedule professional maintenance.

Should the AC fan run all the time?

Not typically. In Auto mode, the fan cycles with cooling. Some homes use Circulate mode for air filtration, but it increases energy use and may reduce comfort if the coil isn’t actively cooling.

How do I stop my AC fan from running constantly?

Set Fan to Auto, replace the filter, open vents and returns, and verify thermostat schedules. If it still runs continuously, a stuck relay, contactor, or control issue may be to blame—call a professional for electrical diagnostics.

Is a constantly running fan bad for my AC?

Yes. Continuous fan operation increases energy consumption and can circulate warm, humid air when there’s no active cooling. It may also mask deeper issues like frozen coils, low refrigerant charge, or electrical faults.