AC Not Cooling? Safe Checks and Common Causes

Quick answer

When an AC runs but doesn't cool, the cause is often a dirty filter, a thermostat setting, a tripped breaker, or a dirty or blocked outdoor unit. Deeper causes include a failed capacitor, a frozen coil, or a refrigerant leak. Do the safe checks first, then have a technician find the cause.

Key facts

Safe checks	6, no tools needed
Common simple causes	Filter, thermostat, breaker, blocked outdoor unit
Common part failure	Capacitor or contactor at the outdoor unit
Refrigerant	Handled only by EPA-certified technicians
Before any repair	You hear the price and decide

Why is my AC running but not cooling?

Usually, something is limiting how much heat the system can move: low airflow, a dirty coil, or an outdoor unit that isn't running right. ENERGY STAR notes that dirty coils reduce a system's ability to cool your home and make it run longer.

Sometimes the indoor fan runs but the outdoor unit doesn't. Then you get moving air, but no cooling.

What should I check first when my AC isn't cooling?

If the outdoor unit is humming but its fan isn't spinning, turn cooling off at the thermostat before anything else. Running it that way can damage the compressor.

1Set the thermostat to Cool, with the set point below room temperature and the fan on Auto.
2Replace the thermostat batteries if the screen is blank or dim.
3Check the filter and replace it if it's dirty. ENERGY STAR says a dirty filter slows airflow and makes the system work harder.
4Check the breakers for the indoor and outdoor units. Reset a tripped breaker once. If it trips again, stop.
5Go outside. Is the outdoor unit running, with the fan spinning? Clear away leaves, plants, and anything stored against it.
6Make sure supply vents and return grilles are open and not blocked.

Not sure what's wrong yet?

Start with the safe checks. Many problems are a setting, a breaker, or a filter.

Find your symptom Compare repair vs. replace

Repair first: we fix what can be fixed
You approve the price before work starts
Sources listed for outside facts

What usually causes an AC to stop cooling?

Match what you're seeing to the most likely cause. If it's not a filter, vent, or breaker, it's a technician job.

What you notice	Common cause	Can you check it?
Weak airflow at the vents	Dirty filter or blocked returns	Yes
Indoor fan runs, outdoor unit silent	Tripped breaker, failed contactor, or thermostat wiring	Breaker, yes. Parts, no.
Outdoor unit hums, fan doesn't spin	Failed capacitor or fan motor	No. Leave it for a technician.
Ice on the copper lines or indoor coil	Low airflow or low refrigerant	Check the filter. See our frozen lines page.
Cools a little, never catches up	Dirty coils or low refrigerant	No
Water around the indoor unit	Clogged condensate drain	You can look. See our water leak page.

When should I stop and call a technician about my AC?

Call when the checks are done and the air still isn't cool, or when any check points inside the unit.

Don't open the outdoor unit's electrical panel or try to add refrigerant. Some parts inside can hold a charge even with the power off, and under EPA rules, refrigerant work requires a certified technician.

The breaker trips again.
The outdoor unit hums, buzzes, or clicks but won't start.
You see ice on the lines or coil.
You smell burning.
You hear grinding or screeching from the outdoor unit.

What will a technician check on an AC that isn't cooling?

A technician checks the electrical parts at the outdoor unit first, then refrigerant and airflow. ENERGY STAR notes that too much or too little refrigerant makes a system less efficient. If the charge is low, the fix is finding the leak, not just topping it off.

Capacitor, contactor, and wiring at the outdoor unit
Compressor and outdoor fan motor
Refrigerant charge, and signs of a leak
Indoor and outdoor coils
Blower, airflow, and the condensate drain

Can my AC be repaired, or does it need replacing?

Many cooling problems come down to a small part, like a capacitor or contactor, or a coil cleaning. We fix what can be fixed — and tell you straight when it can't.

If the repair is big, like a compressor or a major leak on an older system, we'll show you the repair price, explain the replacement option, and let you decide. No work starts until you say yes.

Common questions

Is it normal for my AC to run nonstop during a heat wave?

It can be normal for a working AC to run almost nonstop on very hot days and still sit a little above the set temperature. If the air from the vents feels warm, or the house keeps getting hotter, that points to a problem. Our heat wave prep guide covers what to check before hot weather.

Why is my outdoor AC unit not running?

The most common reasons are a tripped breaker, a failed capacitor or contactor, or a thermostat that isn't sending the signal. Check the thermostat and breaker first. The parts inside the unit carry high voltage, so leave those for a technician.

Can I clean my outdoor AC unit myself?

You can clear leaves, weeds, and stored items from around it, which is a safe homeowner step. Cleaning the coil itself is a job for a technician. ENERGY STAR lists coil cleaning as part of a typical maintenance check-up.

Is it safe to keep running an AC that isn't cooling?

Whether it's safe depends on why it isn't cooling. If the outdoor unit hums without its fan spinning, or you see ice, turn cooling off to avoid more damage. If airflow is just weak, change the filter and watch it. When in doubt, turn it off and have it checked.

Does wildfire smoke affect my AC?

Wildfire smoke can load up your filter faster than usual, which cuts airflow and cooling. Check the filter more often during smoky stretches. Our wildfire smoke guide covers filter and fan settings.

What does it mean if my AC turns on and off quickly?

An AC that turns on and off quickly is short cycling. It can come from a dirty filter, a thermostat problem, low refrigerant, or a system too large for the house. See our short cycling page for checks.