AC running but not cooling? What to check first

An air conditioner that runs but blows warm or barely cool air is frustrating, and summer service calls can mean waiting days. Many causes are simple, though. Work through these checks in order before you book a visit.

1. Thermostat settings

Confirm the thermostat is set to Cool, the setpoint is below the current room temperature, and the fan is on Auto rather than On. With the fan set to On, it keeps blowing between cooling cycles, which can feel like warm air. Replace the batteries if the display is dim or blank.

2. The air filter

A clogged filter cuts airflow so badly that the system can't move cool air through the house, and it can make the indoor coil freeze (see step 5). Replace it if it's gray or dense. Our filter guide explains schedules and ratings.

3. Both breakers

Central AC usually has two breakers: one for the indoor furnace or air handler, and a separate, larger double breaker for the outdoor condenser. If the indoor fan runs but the outdoor unit is silent, the outdoor breaker may have tripped. Reset it once. If it trips again, stop and call a technician.

4. The outdoor unit

Go outside while the system is calling for cooling. You should hear the compressor and see the top fan spinning.

• Nothing running: check the disconnect box mounted on the wall near the unit; it may have a pull-out block or switch that's off.
• Humming but fan not spinning: often a failed capacitor or motor. Turn the system off and call a technician. Capacitors can hold a dangerous charge even with power off.
• Running but coils caked with dirt, grass or leaves: the unit can't release heat. Turn the power off at the disconnect, clear debris, and gently rinse the fins from the inside out with a garden hose (no pressure washer). Keep about 2 feet of clearance around the unit.

5. Ice on the lines or coil

Look at the larger, insulated copper line where it enters the indoor unit, or at the outdoor unit. Frost or ice means the indoor coil is frozen. Turn cooling off and set the fan to On to thaw it, which can take several hours. Put towels down, since melting ice produces a lot of water. Then replace the filter and make sure vents are open. If it freezes again with good airflow, low refrigerant is a common reason, and that needs a technician.

6. The condensate drain

Air conditioners remove moisture, which drains through a pipe or a small pump. Many systems have a float switch that shuts the AC off if the drain backs up. Look for water in the drain pan under the indoor unit or a full condensate pump. Clearing a clogged line can restore cooling.

7. Vents and returns

Closed registers and blocked return grilles reduce airflow in the same way a dirty filter does. Open them all and move furniture away from returns.

8. Measure the temperature split

With the AC running for 15 minutes, hold a thermometer at a return grille and then at a supply vent near the indoor unit. A difference of roughly 15 to 20°F generally means the system is cooling normally. Much less points to a problem worth reporting to your technician.

Also keep expectations realistic on extreme days. Residential systems are sized for typical local summer peaks, so during a heat wave an AC may run constantly and still sit a few degrees above the setpoint. Closing blinds on sunny windows helps.

What not to do

• Don't add refrigerant yourself. It requires certification, and low refrigerant means there is a leak to find.
• Don't keep resetting a breaker that trips repeatedly.
• Don't run the AC while the coil is frozen.