Short answer: an AC that runs but doesn't cool is most often low refrigerant from a slow leak, a dirty outdoor condenser coil blocking heat exchange, or a frozen indoor coil caused by restricted airflow. Turn it to "fan only" if you see ice, then call a technician.
Most likely causes, in order
- Low refrigerant. The system isn't designed to "use up" refrigerant — if the level is low, there's a leak somewhere. A recharge alone is a temporary fix; the leak needs to be found and sealed or it comes right back.
- Dirty condenser coil. The outdoor unit needs airflow across its coil to dump heat. A season of dust, cottonwood fluff, or overgrown landscaping pressed against it can measurably reduce cooling capacity.
- Frozen evaporator coil. Usually caused by restricted airflow (a clogged filter, closed vents, or a failing blower motor) — the fix is often airflow, not refrigerant.
- Oversized or undersized system. Less common, but a system mismatched to the home's actual load can run constantly without ever quite catching up on a 100°F+ day.
What to do right now
Check your air filter first — it's free and takes two minutes. If it's clean and the unit still isn't cooling, this is a real diagnostic job for a licensed technician rather than something to keep troubleshooting yourself. See our full emergency AC repair triage for the complete checklist, or our AC repair cost guide for what a repair like this typically costs.
Page last reviewed 2026-09-13.