Why is my AC not cooling? 8 common causes (and fixes)

When your air conditioner is running but the house won't cool down, it's frustrating — especially on a hot San Diego afternoon. Here are the most common reasons an A/C stops cooling, what you can safely check yourself, and when it's time to call a technician.

1. A dirty or clogged air filter

This is the number-one cause we see. A filter packed with dust chokes airflow, so the air that reaches your rooms is weak and barely cool. Check your filter first — if it's dirty, replace it. Do this every 1–3 months in cooling season.

2. Thermostat settings

Make sure it's set to COOL and the fan is on AUTO (not ON, which just circulates air without cooling). Try lowering the setpoint a few degrees to confirm the system kicks on.

3. A tripped breaker or blown capacitor

If the indoor fan runs but the outdoor unit is silent, a tripped breaker or a failed run capacitor may be the culprit. You can reset a breaker once; if it trips again, stop and call a pro — repeated tripping points to an electrical fault.

4. A dirty outdoor condenser coil

The outdoor unit sheds your home's heat. When its coil is caked in dust, leaves, or cottonwood, it can't release heat and cooling drops off. Gently rinsing the exterior can help, but a deep coil cleaning is best left to a technician.

5. Low refrigerant / a leak

If your A/C is low on refrigerant, it can't absorb heat — you may notice ice on the lines or a hissing sound. Refrigerant doesn't get "used up," so low charge almost always means a leak that needs professional diagnosis and repair. This is not a DIY fix.

6. A frozen evaporator coil

Restricted airflow or low refrigerant can freeze the indoor coil into a block of ice — which stops cooling entirely. Turn the system off and let it thaw, then call for service to find the underlying cause.

7. Leaky or disconnected ducts

If cold air is escaping into your attic through leaky ducts, it never reaches your rooms. Uneven cooling from room to room is a common sign.

8. An aging or undersized system

An A/C that's 12–15+ years old, or one that was never sized correctly for the home, may simply be unable to keep up during a heat wave. At that point, repair-vs-replace math matters.

When to call AIM Mechanical

If you've checked the filter, thermostat, and breaker and your home still won't cool, it's time for a professional diagnosis. We offer same-day A/C repair across San Diego County and a 24/7 emergency line — a real local technician, not a call center. See our A/C repair service →

Frequently asked questions