If your AC is running but the house will not cool, check six things first: the thermostat, the breaker, the air filter, the outdoor unit, ice on the coil, and the drain line. Many Phoenix-area service calls end with one of these. Do the checks in order, and stop and call a pro if anything on this list says to.
Time: about 20 minutes. Skill: easy.
Safety first
Never open the panels on your indoor or outdoor unit. Never touch refrigerant lines or wiring. If it is over 100 degrees and you have young children, older adults or pets at home, plan to go somewhere cool while you wait. A dead AC in Arizona is a safety issue, not just a comfort one.
1. Is the thermostat set correctly?
Set it to Cool and the fan to Auto, and set the temperature a few degrees below the room. If the screen is blank or dim, replace the batteries. Wait about five minutes. A thermostat on Heat, or on a schedule that moved the setting up, is the most common cause.
2. Did a breaker trip?
Go to your main electrical panel and look for a breaker that sits in the middle or off position. To reset, push it firmly all the way to Off, then back to On, one time only. Leave the panel cover alone. If it trips again, stop. Repeated tripping means a problem that needs an electrician or an AC company.
3. Is the filter clogged?
Pull the filter and hold it to a light. If you cannot see light through it, replace it. See how to change your AC filter for the steps. A clogged filter cuts airflow and can cause the next problem.
4. Is the outdoor unit blocked or dirty?
Turn the thermostat to Off first. Look at the outdoor unit. Clear leaves, weeds and trash from around it, keeping at least two feet of open space on every side. Cottonwood fluff, dust and dryer lint can coat the fins. You can gently rinse the outside of the fins with a garden hose on low. Do not use a pressure washer, because bent fins reduce airflow. If the fan is not spinning while the unit hums, turn the system off and call.
5. Is there ice on the coil or lines?
Look at the thin copper line going into the indoor unit and at the outdoor unit's larger line. Ice or frost is a warning. Turn the thermostat to Off and set the fan to On so air melts the ice, which can take several hours. Never chip ice off. If it freezes again after a clean filter, you likely have a low refrigerant or airflow problem, and refrigerant work is a pro job only.
6. Is the drain line clogged?
Your AC pulls water out of the air and sends it outside through a drain line. Algae can clog it, and many systems have a safety switch that shuts the AC off when the water backs up. Look for water in the pan under the indoor unit or around the air handler. If you can reach the vertical access pipe near the unit, pouring in a cup of white vinegar can help. If water is pooling, shut the system off and call, so it does not damage the ceiling.
When to call a pro
Call if the system runs but blows warm air after these checks, if you smell burning, hear grinding, see ice come back, or the breaker keeps tripping. Here are local AC companies in the directory, with phone numbers:
Verify a contractor's license at roc.az.gov before you hire. Ask about the cost of a diagnosis before they come out.
Questions people ask
Why is my AC running but blowing warm air?
The common causes are a thermostat set wrong, a clogged filter, a dirty or blocked outdoor unit, a frozen coil, or a refrigerant or electrical problem. The first four you can check yourself. The last two need a pro.
How cold should my AC get the house on a 115 degree day?
On the hottest afternoons many systems cannot reach a low setting, and that is not always a fault. A house that holds steady in the high 70s or low 80s while it is extremely hot outside is different from air that feels warm coming out of the vents. If the indoor temperature keeps climbing, call.
Is it okay to keep running a frozen AC?
No. Running a system with ice on the coil can damage the compressor. Turn it off, run only the fan to thaw it, and find the cause.
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