AC Repair: Common Problems, Likely Causes and What to Check First

Most central AC repairs come down to a short list of parts: a run capacitor, a contactor, the condenser fan motor, a refrigerant leak, a frozen evaporator coil, a clogged condensate drain or, less often, a control board. Warm air, a unit that won't start, ice, water and noises each point to different parts, so diagnose by symptom. Check the few safe things below first, then use the rest of this guide to judge the repair a technician proposes.

What's Wrong With My AC? Match the Symptom to the Likely Cause

This table covers the problems behind most central air repair visits, with the most common cause listed first.

If your symptom involves a burning smell, a breaker that trips again after one reset, or heat that puts someone at risk, read what counts as an AC emergency before anything else.

What Can You Safely Check Before Calling a Technician?

Plenty of no-cooling calls end with a thermostat setting or a tripped breaker. These checks are safe and take about 15 minutes.

1. Thermostat. Confirm it's set to Cool, the setpoint is several degrees below room temperature, and the fan is on Auto. Replace the batteries if the display is blank or faint.
2. Breakers. Most central systems have two: one for the outdoor condenser and one for the indoor air handler or furnace. Reset a tripped breaker once. If it trips again, leave it off. That pattern means a short or a failing motor or compressor, not bad luck.
3. Outdoor disconnect. The gray box on the wall near the condenser has a pull-out block or a lever. Make sure it's fully seated or switched on.
4. Air filter. Pull it and hold it up to a light. If you can't see light through it, replace it. A clogged filter is the most common cause of a frozen coil.
5. Condensate drain and float switch. Standing water in the pan under the indoor unit can trip a float switch that shuts cooling off on purpose. Clearing the line usually brings the system back.
6. Outdoor unit clearance. Remove leaves, grass clippings and anything stacked against the cabinet. You want about two feet of open space around it.

After any reset, leave the thermostat off for five minutes so refrigerant pressures equalize before the compressor tries to start.

What not to touch

Leave the rest to someone with the right meters and training:

• Don't open the condenser's electrical panel. The run capacitor can hold a dangerous charge even with the power off, and the contactor carries 240 volts whenever the disconnect is in.
• Don't add refrigerant or open the refrigerant circuit. Handling refrigerant requires EPA Section 608 certification, and a low charge means a leak that has to be found first.
• Don't push a fan blade to "jump start" a unit or keep resetting a breaker. Both can mask a failing motor or compressor.
• Don't chip ice off the coil. Set cooling to Off, leave the fan On and let it thaw.

The Parts That Fail Most Often and What the Repair Involves

Run capacitor. It gives the compressor and condenser fan motor the boost they need to start and run. Heat wears it down, and a weak one often bulges on top. The tech discharges it, measures it against the rating on the label and replaces it if it's out of tolerance. It's one of the cheapest, quickest repairs on this list.

Contactor. This relay sends power to the outdoor unit when the thermostat calls for cooling. Its contacts pit and burn over time (insects wedged between them do the same), causing buzzing, chattering or a unit that won't start. Replacement is quick and inexpensive.

Condenser fan motor. If the outdoor fan doesn't spin, the compressor overheats and trips on its internal overload. A worn motor often squeals or grinds first. The tech rules out the capacitor before condemning the motor, since the two look identical from outside.

Refrigerant leak. An AC doesn't use up refrigerant, so a low charge means a leak, often at a coil, a flare fitting or a service valve. A proper repair means locating it (electronic detector, bubbles or a nitrogen pressure test), fixing it, pulling a vacuum and weighing in the correct charge. Topping off without finding the leak is a recurring bill, not a repair.

Frozen evaporator coil. Ice is a symptom. The cause is usually restricted airflow (filter, closed registers, a dirty coil, a weak blower) or low refrigerant, and the coil has to thaw, sometimes for several hours, before anyone can diagnose it.

Short cycling. Also a symptom rather than a part. Rule out a clogged filter and a thermostat sitting in sun or near a supply register first. If the system ran short cycles from the day it was installed, it may be oversized for the house.

Clogged condensate drain. Algae slime blocks the line, and the pan overflows or the float switch shuts the system off. Our guide to AC leaking water inside shows how to clear it yourself.

Control board. The board in the furnace or air handler runs the blower and passes the thermostat's call outdoors. Surges, heat and age kill boards, but a bad thermostat, a broken wire or a tripped safety switch can look the same, so diagnosis takes care.

Compressors fail too, but less often than the parts that start and protect them. If a technician says the compressor is the problem, see AC compressor repair vs replacement for what proper testing looks like before you approve anything.

What Happens During an AC Repair Visit

A thorough diagnostic follows a set order. The tech confirms the thermostat is calling for cooling, checks voltage at the outdoor unit, tests the capacitor and contactor, and watches the compressor and fan start. With the system running, they measure the temperature drop across the indoor coil and read the refrigerant charge using superheat or subcooling. They also check the filter, drain and blower.

You should get a plain explanation and a written price before work starts. Techs carry common capacitors, contactors and universal fan motors, so those repairs usually happen the same visit. Coils, compressors and brand-specific boards may need ordering.

How Much Does AC Repair Cost?

The part that failed matters more than anything else. Roughly from cheapest to most expensive:

• Low end: capacitor, contactor, drain clearing, thermostat issues, a tripped float switch.
• Middle: condenser fan motor, blower motor, control board, finding and sealing an accessible leak plus recharging.
• High end: evaporator or condenser coil replacement, and the compressor, which is often the single most expensive repair on a central system.

Other factors move the bill: the diagnostic fee and whether it's credited, after-hours rates, your refrigerant type (R-22 is scarce and costly), attic or crawlspace access, parts warranty status and local labor rates. For typical ranges, see AC repair cost by type of repair.

Should You Repair or Replace Your AC?

• Repair when the system is under about 10 years old, the failure is a single electrical part or a drain issue, and the unit has otherwise been reliable.
• Weigh it carefully when the system is 10 to 15 years old and the repair is in the middle or high tier. The common 50 percent rule says that if a repair costs more than half the price of a comparable new system, replacement is usually the better value.
• Lean toward replacement when the system is past 15 years (central ACs typically last about 15 to 20), runs on R-22 (US production ended January 1, 2020), needs an out-of-warranty compressor or coil, or has needed several repairs in two seasons.

Comfort counts too. A system that never kept up may be undersized or poorly installed, and repairs won't fix that. If you're leaning toward replacement, our AC installation guide explains what a proper install involves.

How to Vet an AC Repair Contractor

Questions to ask before approving the work:

• What exactly failed, and how did you confirm it? Can you show me the reading or the part?
• Is the diagnostic fee credited toward the repair?
• Is this price written and fixed, or an estimate that could change?
• Is the replacement part OEM or universal, and what warranty covers parts and labor?
• If refrigerant is involved, where is the leak, and will you repair it or only recharge?
• Is the equipment still under manufacturer warranty, and will you handle the claim?

Red flags:

• A recommendation to replace the system before any testing.
• Adding refrigerant without looking for a leak, especially more than once.
• Pressure to decide on the spot, or a price that only holds "today."
• No license number, no written quote, or reluctance to put the diagnosis in writing.
• Refusing to leave the failed part with you.

Get a second opinion on any high-tier repair or replacement recommendation. Our guide to choosing AC repair companies covers licensing and reviews. Once the system is fixed, what a professional AC tune-up includes explains how annual maintenance catches weak capacitors and slow leaks early.

Frequently Asked Questions

What is the most common AC repair?

A failed run capacitor is the most common reason a central air conditioner stops cooling. It's one of the cheaper repairs, but a capacitor can hold a dangerous charge even with the power off, so leave the testing and replacement to a technician.

Why is my AC running but not cooling the house?

If the indoor fan runs but the air is warm, the outdoor unit often isn't running at all, usually because of a bad capacitor, a failed contactor or a tripped breaker. If both units run and the air is only slightly cool, a dirty filter, a frozen coil or a refrigerant leak is more likely. Our AC not cooling troubleshooting guide walks through each case.

Can I reset my AC myself?

Yes, within limits. Reset a tripped breaker once, then leave the thermostat off for five minutes before turning it back on. If the breaker trips again, leave it off. Repeated resets can damage the compressor or hide an electrical fault.

How long does a typical AC repair take?

A capacitor, contactor or drain clearing is usually done in one visit, often within an hour or two of diagnosis. Leak repairs and compressor or coil replacements can take a full day, or a second visit if parts must be ordered.

Is it safe to keep running an AC that is not cooling properly?

Not for long. Running with a frozen coil, low refrigerant or a humming outdoor unit that won't start can overheat and damage the compressor. Switch cooling off until the cause is found, and use fan-only mode if you see ice on the coil or lines.

Who should I call to fix my AC?

A licensed HVAC contractor, since refrigerant work requires EPA Section 608 certification. Renters should contact the landlord first. Owners with a home warranty or a recent installation should check that coverage first, because an outside contractor may not be reimbursed. An electrician is only the right call for the house wiring or panel feeding the unit. The vetting section on this page lists what to check before you hire.