An air conditioner that runs but doesn’t cool almost always has one of five problems: a dirty filter or blocked airflow, a thermostat or power issue, a dirty or obstructed outdoor unit, a refrigerant leak, or a failed electrical part like the outdoor unit’s capacitor. The first three you can check yourself in ten minutes; the last two need a technician. Start with the checks below in order, because they rule out the cheap causes and tell the contractor something useful when you call.
Check these first (10 minutes)
Thermostat. Set to Cool, setpoint below room temperature, fan on Auto. Replace the batteries if it has them. A blank screen means power or batteries.
Filter. Pull it. If you can’t see light through it, replace it and give the system an hour. A clogged filter starves the indoor coil of airflow, which drops cooling and can freeze the coil.
Breakers and switches. Check the breaker for the outdoor unit and the air handler; a tripped breaker can be reset once, and if it trips again, stop and call (why breakers trip). Check the disconnect box by the outdoor unit and the switch on or near the air handler; both get knocked off.
Outdoor unit. Is the fan spinning? Is the coil caked with grass, dirt or cottonwood fluff? Is anything blocking airflow within two feet? Gently rinse a dirty coil from the inside out with a hose (power off first). If the fan isn’t spinning but the unit hums, that’s usually a capacitor; call.
Ice. Look at the copper lines at the outdoor unit and, if you can, the indoor coil. Ice or frost anywhere means stop: set the system to Off and the fan to On for a few hours to thaw, then read AC freezing up.
Likely causes, ranked
| Cause | How common | Who fixes it | Typical cost |
|---|---|---|---|
| Dirty filter / restricted airflow | Very common | You | $10–$40 filter |
| Dirty outdoor coil | Very common | You (rinse) or tech (chemical clean) | $0–$250 |
| Failed capacitor (outdoor fan or compressor won’t start) | Very common | Technician | $130–$400 |
| Refrigerant leak (low charge) | Common on older systems | Technician; find and fix the leak, then recharge | $600–$1,800+ |
| Failed contactor or fan motor | Common | Technician | $150–$800 |
| Frozen indoor coil (from any of the above) | Common | Thaw, then fix the cause | — |
| Thermostat wiring or failure | Occasional | Technician | $150–$500 |
| Compressor failure | Uncommon; usually the end result of another problem | Technician; often a replace-the-system decision | $1,100–$4,650 |
| Undersized system / duct problems (never cooled well) | Occasional | Contractor; sizing and duct evaluation | Varies |
Costs from the AC repair cost guide.
Warm air vs “not keeping up”
There’s a difference between air that’s warm at the vents and air that’s cool but the house won’t reach setpoint. Warm air points to the outdoor unit not running (power, capacitor, contactor, compressor) or a badly frozen coil. Cool-but-not-enough on hot afternoons points to airflow, a dirty coil, low refrigerant, leaky ducts, or an aging system that’s lost capacity. In North Carolina’s humidity, a house that’s cool but clammy is its own problem, usually oversizing or a system that isn’t running long enough.
When to call
Call now if: the outdoor unit hums but doesn’t start, a breaker trips twice, you see ice, you smell burning, or you have infants, older adults or anyone with a heart or respiratory condition in the house on a hot day (what counts as an emergency). Otherwise, do the checks above, note what you found, and book a diagnostic. When the technician arrives, ask what specifically failed and how they tested it. If the diagnosis is a refrigerant leak, coil or compressor on a system past 10–12 years, get a replacement quote alongside the repair (repair or replace).
Every contractor on the North Carolina registry shows the license status we found; the city pages list which ones run 24-hour service.