Your AC is running, you can hear it, but the house will not cool. Before you call anyone, here are the eight most common reasons - starting with the ones you can fix yourself in five minutes.
1. A clogged air filter
The number-one cause and the cheapest fix. A filter caked in dust chokes airflow, so the system runs without cooling. If it is grey, replace it. Do this every 1-3 months.
2. The thermostat
Check it is on Cool, set below room temperature, and on Auto not On - On runs the fan constantly, blowing room-temp air between cooling cycles. Dead thermostat batteries cause odd behavior too.
3. A dirty outdoor condenser
The outdoor unit dumps your home heat outside. Buried in leaves or grass clippings, it cannot. Turn off the power and gently rinse the fins with a hose. Keep two feet of clearance around it.
4. A blocked vent
Furniture over a return vent, or too many closed room vents, starves the system of airflow. Open them up.
5. A frozen evaporator coil
Restricted airflow or low refrigerant can ice over the indoor coil. Switch to Fan only for a few hours to thaw it, then fix the cause - often the filter.
6. Low refrigerant (a leak)
Weak cooling plus hissing, or a coil that keeps freezing, can mean low refrigerant - which means a leak. This is a licensed-tech job; refrigerant is not a DIY top-up.
7. A failing capacitor or fan motor
If the outdoor fan will not spin, or you hear a hum then a click, the start capacitor may be dead. Cheap part, but it is inside a high-voltage unit - call a pro.
8. An aging or undersized system
If your AC is 15+ years old or was never sized right, it may simply lose the fight on the hottest days. A tech can confirm before you spend on a replacement.
When to call an HVAC pro
Filter, thermostat, vents, and a gentle condenser rinse are safe DIY. Anything involving refrigerant, electrical parts, or the sealed system needs a licensed HVAC pro. Post your job on Handiro and local pros respond, usually within the hour - free to post.