Why is my AC running but not cooling?

By Dana Whitlock. Updated October 2026. Repair or replace?

An AC that runs all afternoon and never cools the house usually has one of three problems. Not enough air is moving across the indoor coil, the outdoor unit can't shed heat, or the system is short on refrigerant. The first two often have causes you can fix yourself. The third always needs a technician, because DOE is clear that topping off a leaking system doesn't solve anything.

Likely causes

A dirty air filter Safe to check

DOE says replacing a dirty, clogged filter can lower an air conditioner's energy use by 5 to 15 percent, and it recommends checking filters every month or two in cooling season. A badly clogged filter can cut airflow enough that the house never cools.

Outdoor unit blocked or dirty Safe to check

The condenser outside has to move heat into the yard. DOE advises keeping it clear of debris and trimming plants back at least 2 feet. Grass clippings, leaves or a stack of patio furniture can choke it.

Outdoor unit not running Safe to check

If the indoor fan blows but the outdoor fan and compressor are silent, the house gets moving air with no cooling in it. A tripped breaker or outdoor disconnect is the simple version. A failed contactor or capacitor is the technician version.

Low refrigerant from a leak Pro job

DOE notes low refrigerant means the system was undercharged at install or it leaks, and that a trained technician should fix the leak, test it and charge to specification. Refrigerant work requires certification.

A frozen or dirty indoor coil Pro job

Ice on the refrigerant lines or a dirty evaporator coil blocks heat transfer. DOE recommends checking the evaporator coil every year and cleaning it as needed. Getting to it means opening the air handler, which is technician work.

Safe things to check first

• Set the thermostat to Cool, fan on Auto, and the setpoint a few degrees below room temperature. Replace its batteries if the display is dim.
• Replace the air filter if it is gray or you cannot see light through it.
• Go outside and listen. If the outdoor unit is silent, check the breaker and reset it once.
• Clear leaves, grass and clutter from around the outdoor unit and keep 2 feet open on all sides.
• Look for ice on the copper line at the outdoor unit. If you see it, turn the system off and let it thaw.

When to stop and call

Call a licensed HVAC technician if the outdoor unit hums but does not run, if the breaker trips again, if you see ice, or if the house still will not cool after a clean filter and clear outdoor unit. NFPA's electrical safety guidance lists frequent breaker trips and burning smells as reasons to call a qualified electrician; with an AC, the HVAC technician is usually the first call. Do not open the outdoor unit's electrical panel.

Repair or replace?

Start with its age. Type the serial from the label:

Filter and clearance fixes cost nothing. Electrical parts like a capacitor or contactor are modest repairs. A refrigerant leak on an older system is the expensive fork in the road. DOE's 2016 analysis puts central AC lifetimes at about 24 years in northern and hot-dry regions and about 18 in hot-humid ones, and inspector charts list as little as 7 to 15 years. Date your system with the age decoder above. Near the end of those ranges with a leak, compare against the replacement range above.