AC not cooling: safe checks and how to stay cool

Updated October 26, 2026. General information only, not professional advice.

When the air conditioner stops cooling on a hot day, the house can heat up fast. Before anything else, make sure the people in the home are safe from the heat. Then try the simple checks below. Many of them take only a few minutes, and none of them require opening up the equipment.

Watch for heat illness first

Heat can be dangerous, especially for older adults, young children, people with health conditions and pets. Ready.gov lists these warning signs.

Heat exhaustion: heavy sweating, paleness, muscle cramps, tiredness, weakness, a fast or weak pulse, dizziness, headache, fainting, nausea or vomiting. Move to a cooler place, loosen or remove extra clothing, and sip water or a sports drink. Ready.gov says to call a healthcare provider if symptoms get worse or last more than an hour.

Heat stroke: a body temperature above 103°F, red, hot and dry skin with no sweat, a rapid strong pulse, dizziness, confusion or passing out. This is a medical emergency. Call 911 or get the person to a hospital right away.

Stay cool while the AC is down

Do not count on a fan alone. Ready.gov says not to rely on a fan as your main cooling device.
Close blinds and curtains on sunny windows.
Drink plenty of fluids and wear light, loose clothing.
Avoid using the oven and other heat-producing appliances.
Spend the hottest hours somewhere with air conditioning, such as a library, store, or a friend's home. Ready.gov suggests finding a cooling center if your home is too hot.
Never leave a person or pet in a parked car.

Check the thermostat

Make sure it is set to Cool, not Heat, Fan or Off.
Set it a few degrees below the current room temperature.
Replace the batteries if the screen is blank or dim.
Check that a schedule or hold setting is not overriding you.

Give the system several minutes to respond after a change.

Check the power

Breakers. Most central systems have two breakers, one for the indoor unit and one for the outdoor unit. Reset a tripped breaker once. If it trips again, leave it off and call for repair, since repeated tripping points to an electrical problem.
Outdoor disconnect. There is usually a shutoff box on the wall near the outdoor unit. Make sure it has not been switched off or pulled.
Indoor switch. The furnace or air handler often has its own service switch. Make sure it is on.

If power is out to the whole house, Ready.gov's power outage advice applies, including never running a generator indoors.

Check the air filter

A dirty filter blocks airflow and can make the system cool poorly or ice up. ENERGY STAR suggests inspecting, cleaning or changing filters once a month.

Turn the system off at the thermostat.
Pull out the filter. If you cannot see light through it, replace it.
Match the size on the frame and point the arrow toward the unit.

Look at the outdoor unit and the drain

From the outside, with the power off at the disconnect:

Clear leaves, grass clippings and debris from around the unit.
Make sure plants or furniture are not crowding it.
Do not open the unit or touch wiring.

Inside, look near the indoor unit for water. ENERGY STAR notes that a plugged condensate drain can cause water damage in the house. Some systems have a safety switch that shuts cooling off when the drain backs up. If you see standing water, turn the system off and call for service.

If you see ice

Ice on the indoor coil or on the copper lines outside often points to poor airflow or a refrigerant problem. Turn cooling off and run the fan only, or leave the system off, so the ice can melt. Replace the filter if it is dirty. If the ice comes back, a technician needs to find the cause. Refrigerant work is a job for a trained technician, not a homeowner.

When to call a pro

Call an HVAC technician if the thermostat, power and filter all check out and the system still is not cooling, if a breaker keeps tripping, if you see ice that returns, if water is leaking, or if the unit hums, buzzes, clicks or smells like burning. If anyone in the home is showing signs of heat illness, get them cool and get medical help first.

Before you hire anyone, verify that they hold any license your state requires for HVAC work, and ask for proof of insurance. Get a written estimate before work starts, and ask whether the price includes parts, labor and any refrigerant. ENERGY STAR also recommends a yearly pre-season check-up, with cooling checked in spring, which can catch problems before the hottest days.