Why Is My AC Not Working?

Why would my AC suddenly stop working?

An AC that quits all at once almost always lost power, lost its signal to run, or froze up. The four most common sudden failures are a tripped breaker or blown fuse, a thermostat that switched off or lost battery power, a dirty filter choking airflow, and a frozen evaporator coil that ices over and blocks the air. Any one of these will stop cold air without the compressor itself being broken, which is good news because they are the cheap end of the problem list.

The next tier up is electrical and mechanical: a failed run capacitor (the single most common part failure), a bad contactor, a burned-out fan motor, or a tripped safety float switch in a clogged drain line. A failed capacitor usually shows up as a unit that hums or buzzes but will not start the fan or compressor. These are still repairs, not replacements, but a few cross into territory you should not DIY.

The expensive failures are a dead compressor and a refrigerant leak. A unit low on refrigerant will run constantly, ice up, and never cool, and a compressor that has failed will trip the breaker or sit silent. Both need a licensed tech, both cost real money, and both are worth diagnosing carefully before you decide between repairing and replacing the system.

What is the first thing to check when the AC stops working?

Check the thermostat first, then the breaker, then the air filter, in that order. Set the thermostat to Cool and drop the target temperature five degrees below the room reading; if it has batteries, replace them. A blank or unresponsive screen, or a thermostat left on Heat or Off, is the single most common reason a homeowner thinks the AC is broken when it is not.

Next, go to the electrical panel and look for a tripped breaker on the AC circuit, which sits halfway between on and off. Flip it fully off, then back on. If the breaker trips again right away, stop and call a pro, because a repeated trip means a short or an overloaded motor, not a nuisance trip. Also check the outdoor disconnect box on the wall next to the condenser and make sure it is seated.

Then pull the air filter and hold it up to the light. A filter so clogged you cannot see light through it starves the system, freezes the coil, and shuts cooling down, and it is the most overlooked cause of a weak or dead AC. Replace a disposable filter or wash a reusable one, and plan to change filters every 30 to 90 days. While you are at it, check that the condensate drain line is not clogged, since a full drain pan trips a safety switch that cuts the system off.

How do I reset my air conditioner?

To reset a central AC, turn it off at the thermostat, switch off its breaker for 30 seconds, turn the breaker back on, then wait a few minutes before setting the thermostat back to Cool. This power-cycle clears a tripped internal control and is the safe version of a reset that works for most units. Many systems also have a small recessed reset button on the outdoor condenser; press and hold it for a few seconds if power-cycling alone does not bring the unit back.

After any reset or power interruption, wait at least three minutes before the compressor restarts. Modern thermostats and control boards build in this delay on purpose to protect the compressor, so a unit that seems dead for a couple of minutes after a reset is often just waiting out its safety timer. If it has not kicked on after five minutes with the thermostat calling for cool, the reset did not fix the underlying problem.

A reset fixes a glitch, not a broken part. If the AC trips off again within an hour, freezes up, or hums without starting, the reset bought you nothing and you are back to diagnosing a real fault: a clogged filter, a failed capacitor, or low refrigerant. Do not keep resetting a unit that trips its breaker repeatedly, because you are just stressing a motor that is already in trouble.

Why is my AC running but not blowing cold air?

An AC that runs but blows warm or weak air is usually iced over, starved of airflow, or low on refrigerant. The most common cause is a frozen evaporator coil, often from a dirty filter or a failing blower, which turns the indoor coil into a block of ice that blocks all airflow. If you see frost on the copper lines or the indoor coil, turn the system off and run just the fan for an hour or two to thaw it, then replace the filter before running cooling again.

If the coil is clear and the filter is clean, the next suspects are the outdoor unit and the refrigerant charge. Check that the outdoor condenser fan is spinning and the coils are not buried in leaves, grass clippings, or cottonwood fluff, because a smothered condenser cannot dump heat and the air inside never gets cold. A gentle rinse of the outdoor coil with a garden hose (power off) often restores cooling on a dirty unit.

If airflow is fine and the unit still will not cool, suspect low refrigerant, which means a leak. Topping off refrigerant is a job for an EPA-certified technician; it is illegal to buy and handle most refrigerants without that certification, and adding more without fixing the leak just wastes money. A unit that runs constantly, ices up, and never reaches temperature is the classic low-charge pattern. Do not chase it yourself; get a tech to find the leak.

Which AC problems should I not try to fix myself?

Leave anything involving refrigerant, mains-voltage wiring, or a stored electrical charge to a licensed pro. Refrigerant work is regulated under EPA Section 608 and is illegal to do without certification, so a low charge, a hissing leak, or a compressor problem is never a DIY job. The same goes for replacing a contactor, rewiring a 240-volt circuit, or working inside the breaker panel, where a mistake is a shock or fire risk.

The capacitor is the gray area. It is a cheap part and a common failure, but it holds a dangerous charge even with the power off, so if you attempt it you must cut power at the thermostat, breaker, and outdoor disconnect, then discharge the capacitor before touching a wire. We walk through the safe way to do it in how to change an AC capacitor, and if any part of that makes you uneasy, hire it out.

When repairs start stacking up on an old system, weigh fixing against replacing. A rough rule: multiply the unit's age in years by the repair quote, and if it clears $5,000, lean toward replacement. If you do replace it, size the new system properly with the BTU calculator instead of matching the old tonnage, because an oversized AC short-cycles, never dehumidifies well, and wears its parts out faster.

When should I call a pro, and what will it cost?

Call a pro when the breaker trips repeatedly, the unit is low on refrigerant, the compressor is dead, or any safe check did not bring cooling back. A standard HVAC diagnostic visit runs about $75 to $200, and that fee usually rolls into the repair if you go ahead. Paying for a diagnosis is worth it when you cannot tell whether you are facing a $30 part or a failed compressor.

Common repair costs give you a sense of scale before the truck arrives. A capacitor runs about $150 to $400 installed, a contactor $150 to $350, a condenser fan motor $300 to $700, and a refrigerant leak repair $200 to $1,500 depending on where the leak is. A failed compressor is the big one at roughly $1,300 to $2,800, which is often the point where a new system makes more sense than a repair.

Get the system looked at as a whole if it is more than 10 to 12 years old and this is not its first failure of the season. A good tech will check the capacitor, the contactor, the refrigerant charge, and the motor amp draw in one visit, which keeps you from paying for a string of single-part fixes on a unit that is near the end anyway.

Frequently asked questions