Fan Running Constantly (AC) — Causes, Fixes & When to Call

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 be caused by thermostat settings, airflow restrictions, or electrical faults like a stuck relay or contactor. Below is a guide with practical checks, deeper explanations, and trusted resources for more information.

Quick 60-Second Triage

• Thermostat Mode: Ensure the fan is set to Auto, not On.

• Setpoint: Lower temperature 3–5°F below room temp.

• Filter: If clogged or gray, replace immediately.

• Outdoor Unit: See if only the blower runs or both indoor and outdoor fans.

Common Reasons the AC Fan Runs Constantly

1. Thermostat Fan Set to “On”

When the thermostat is on On, the fan runs continuously. Switch to Auto so the fan cycles with cooling.

2. Poor Airflow or Dirty Filter

Restricted airflow makes the system work longer to reach temperature. Replace the air filter every 1–3 months.

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 are electrical components that should be checked by a professional.

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.
🔗 Learn more: Honeywell Thermostat Support – Fan Won’t Turn Off

6. Coil or Refrigerant Problems

When your system can’t reach setpoint due to a dirty coil, refrigerant leak, or frozen evaporator, it may run endlessly.

What You Can Try Yourself

1. Set thermostat Fan to Auto.

2. Replace dirty filters.

3. Make sure vents and returns are open.

4. Turn off any “Circulate” or “Humidity” fan settings in smart thermostats.

5. Verify outdoor condenser area is clear of leaves and debris.

When to Call a Professional

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

• A stuck relay, contactor, or control board failure.

• A wiring short between R and G.

• Poor cooling due to low refrigerant or airflow issues.

• Coil icing or mechanical wear.

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

Prevent Continuous Operation

• Replace filters regularly.

• Schedule annual tune-ups for coil cleaning and electrical checks.

• Educate family on thermostat modes (Auto vs. On).

• Seal leaky ducts to reduce runtime.

Helpful Outbound Resources

• ENERGY STAR – Air Conditioning Efficiency Tips

• ASHRAE – Understanding HVAC System Operation

• HVAC.com – Why Your AC Runs Constantly

• U.S. Department of Energy – Cooling System Guide

• Carrier Residential Support – Troubleshooting Fan Problems

Each of these sources offers trusted information about maintenance, thermostat operation, and energy efficiency.

Contact GoTech Air Conditioning & Heating

If your AC is running constantly or the fan won’t shut off, our certified technicians can help.
We provide same-day diagnostics, full system tune-ups, and control board repairs across Orange County, CA.

Call (949) 228-6242 or Request Service Online to schedule your appointment today.
GoTech Air Conditioning & Heating — Local, family-owned HVAC experts with 5+ years of trusted service.