1. A clogged air filter
The number one cause, by far. A filter packed with dust chokes airflow, so the cold coil can't hand its cold to the air moving past it. The AC runs and runs, the house barely cools, and your power bill climbs. In Miami, with the AC running nearly year-round, filters clog faster than the box says. Check it first — if you can't see light through it, replace it.
2. A dirty outdoor coil
Your outdoor unit has to dump the heat it pulled from your house. If its coil is coated in dirt, grass clippings, or leaves, it can't. Gently rinse the fins with a garden hose (power off first, no pressure washer). This alone can bring a struggling system back.
3. Low refrigerant from a leak
Refrigerant doesn't get "used up" — if it's low, it leaked. Signs: the AC cools a little but never enough, ice on the copper lines, or a hissing sound. This one needs a licensed tech, because the fix is finding and sealing the leak, not just topping off gas that will leak out again.
4. A frozen evaporator coil
Weak airflow or low refrigerant can freeze the indoor coil into a block of ice, and ice blocks airflow completely. If air barely comes out of the vents, switch the system to FAN ONLY for a few hours to thaw it, then see our guide on frozen AC lines.
5. A failing capacitor
The capacitor gives the compressor its starting kick. When it weakens, the outdoor fan may spin while the compressor never starts — so air moves but it isn't cold. You may hear a hum or a click from the outdoor unit. It's one of the cheapest AC repairs there is when caught early.
6. Thermostat problems
If the settings don't seem to matter, the brain may be the problem, not the body. See our thermostat guide — it's a two-minute check.
7. A system too small (or too old) for the job
If your AC has cooled poorly every summer, or it's 12–15+ years old and getting worse, the honest answer may be capacity or age. We'll tell you straight whether a repair buys you real time or just delays the inevitable.