AC running but not cooling? The ten-minute check before you call anyone

Each check takes a minute, and the order matters because the easy ones are also the common ones.

1. 1Thermostat. Mode on Cool, fan on Auto, setpoint at least three degrees below room temperature. If the screen is blank or dim, change the batteries before anything else.
2. 2Power. Check the breaker panel for a tripped breaker on the air handler or the condenser (there are usually two). Reset once. Check the light switch next to the furnace or air handler in the attic or closet; it gets bumped. Check the outdoor disconnect box on the wall by the condenser has not been pulled.
3. 3Filter. Pull it. If you cannot see light through it, that is the problem or a large part of it. Replace it and give the system an hour.
4. 4Float switch. Look in the drain pan under the indoor unit. Standing water means the drain line is clogged and the safety switch has shut the system down on purpose. Clearing the line restarts it. Our drain line guide shows how.
5. 5Vents. Open every supply register and move furniture off the returns. Closing vents to push air elsewhere does the opposite in a ducted system.
6. 6Outdoor unit. With the thermostat calling, the fan on top should be spinning and the air coming off it should be warm. If it hums and sits still, the capacitor has likely failed. Do not spin it with a stick; turn the system off and call.
7. 7Ice. Look at the larger copper line where it enters the house and at the indoor coil if you can see it. Any frost means stop: thermostat off, fan on, and read our freezing guide before you do anything else.

Residential systems in DFW are designed around an outdoor temperature of about 100 degrees. On a 105-degree afternoon with a 140-degree attic, a healthy system may only hold the house in the high 70s and run without stopping. That is not a failure. The test is the temperature drop across the coil: air coming out of the supply registers should be roughly 15 to 20 degrees cooler than the air going into the return. A cheap probe thermometer in a register tells you in two minutes.

If the split is under 10 degrees after a clean filter, something is wrong with airflow or refrigerant and it is a service call. If the split is fine and the house is still warm, the problem is usually ductwork, attic heat or the house itself, and a technician with a duct leakage tester finds it faster than a replacement salesman does.

Say what you checked and what you saw: whether the outdoor fan spins, whether there was water in the pan, whether there was ice, and how old the system is (the data plate on the outdoor unit has a manufacture date). A tech who hears that walks in with the right part on the truck.