1. Check the thermostat
Make sure it's set to Cool (not Fan or Auto-heat) and a few degrees below the room temperature. Replace the batteries if it has them. A blank or flickering screen can stop the system from calling for cooling.
2. Replace the air filter
A clogged filter starves the system of airflow. Weak airflow can make the indoor coil freeze, and a frozen coil can't cool. If the filter looks gray or you can't see light through it, replace it.
3. Look for ice
Ice on the copper line at the outdoor unit or on the indoor coil means the system is freezing up. Turn cooling off and run only the fan for a few hours to thaw it. If it freezes again after a new filter, call a pro: low refrigerant or a blower problem is likely.
4. Check the breaker and outdoor unit
With the AC on, go outside. The big fan on top should be spinning. If it's still, check the breaker in your panel and the shutoff box next to the unit.
If the unit hums but the fan doesn't turn, the capacitor has probably failed. It's a common, fairly inexpensive repair, but capacitors store a dangerous charge. Don't open the unit yourself.
- Clear leaves, grass and clutter at least two feet around the unit.
- With the power off, gently rinse a dusty outdoor coil with a garden hose.
- Reset a tripped breaker once. If it trips again, leave it off and call.
5. Check the vents and returns
Closed supply vents and blocked return grilles cut airflow. Open the vents and move furniture or rugs off the returns. If air is weak at every vent, the problem may be the blower or ductwork.
6. Consider the weather
On extreme-heat days, many systems can only keep the house about 20 degrees below the outdoor temperature. If the air from the vents is cool and the house slowly warms in the late afternoon, the system may be working as designed.