If your air conditioner is running but your house isn’t getting cool, the cause is usually one of a dozen common problems, and several of them take less than 10 minutes to check yourself. The most frequent culprits are a dirty air filter, a wrong thermostat setting, a tripped breaker, and a dirty or blocked outdoor unit.
This guide walks through all 12 causes in the order a technician would check them: easiest and most likely first, expensive repairs last. For each one you’ll see the symptoms, what you can safely do yourself, and when it’s time to call a licensed HVAC pro.
Safety first: turn off power to the system at the thermostat and the breaker before opening any panel. The outdoor unit contains capacitors that can hold a dangerous electrical charge even with the power off. Never open the electrical compartment, and never handle refrigerant yourself. Refrigerant work legally requires an EPA Section 608 certified technician.
Quick checks (5 minutes)
Before you dig in, run through these. They solve a surprising share of “AC not cooling” calls.
- Thermostat: is it set to COOL (not HEAT, AUTO-fan or OFF), and is the setpoint at least 3 to 5 degrees below the current room temperature?
- Batteries: if the thermostat screen is blank or dim, replace its batteries.
- Breakers: check the main electrical panel for a tripped breaker labeled AC, condenser or air handler/furnace.
- Outdoor disconnect: the box on the wall next to the outdoor unit should be switched on or have its pull-out block inserted.
- Air filter: pull it out. If you can’t see light through it, replace it.
- Vents: make sure supply registers are open and not blocked by furniture or rugs.
If all six check out and the AC still isn’t cooling, use the table below to narrow it down.
Symptoms at a glance
| Symptom | Most likely cause | DIY? | Typical repair cost* |
|---|---|---|---|
| Weak airflow from vents | Dirty air filter | Yes | $10 to $40 (filter) |
| Outdoor unit off, indoor fan running | Tripped breaker, failed capacitor or contactor | Breaker only | $150 to $450 |
| Outdoor unit hums but fan doesn’t spin | Bad capacitor or fan motor | No | $150 to $700 |
| Ice on the indoor coil or copper lines | Low airflow or low refrigerant | Partly | $0 to $1,500+ |
| Warm air, hissing or bubbling sound | Refrigerant leak | No | $300 to $1,500+ |
| Water around the indoor unit, AC shuts off | Clogged condensate drain | Yes | $0 to $250 |
| Runs constantly, house never reaches setpoint | Dirty condenser coil, undersized system, duct leaks | Partly | $100 to $2,000+ |
| Turns on and off every few minutes | Short cycling (several causes) | Partly | $100 to $600 |
| Some rooms cool, others hot | Duct problems or closed dampers | Partly | $0 to $2,000+ |
*Costs are typical U.S. ranges for parts and labor and vary widely by region, equipment and contractor. See our AC repair cost guide for details.
1. The thermostat is set wrong or failing
A thermostat in the wrong mode is the single most common “my AC isn’t working” call. Smart thermostats add new failure points: schedules, eco modes, and lost Wi-Fi.
Symptoms: the system doesn’t start at all, starts and stops at odd times, or the screen is blank.
What you can do:
- Set the mode to COOL and the fan to AUTO. With the fan set to ON, the blower runs even when the compressor is off and pushes room-temperature air, which feels like the AC isn’t cooling.
- Lower the setpoint 5 degrees and wait 5 minutes. Most systems have a built-in delay of 3 to 5 minutes to protect the compressor.
- Replace the batteries, even if the screen looks fine.
- On smart thermostats, check for an active schedule, eco or away mode, or a pending “delayed” message. Many units wait for the compressor protection timer before starting.
- Make sure the thermostat isn’t in direct sun or next to a heat source like a lamp or TV, which makes it read high.
Call a pro if: the thermostat has power but the system never responds. That can point to a wiring or control board problem.
2. The air filter is clogged
A dirty filter chokes airflow across the indoor coil. Less air means less heat removed from your home, and in bad cases the coil drops below freezing and ices over (see cause 6).
Symptoms: weak airflow from the vents, the system runs longer than usual, dust buildup around registers.
What you can do: replace the filter. Standard 1-inch filters usually need changing every 1 to 3 months, more often with pets, allergies or construction dust. Match the size printed on the old filter’s frame and install it with the arrow pointing toward the blower (away from the return grille). A MERV 8 to 11 filter is a good balance of filtration and airflow for most homes. Very restrictive filters can reduce airflow on systems that weren’t designed for them.
3. A breaker tripped or the outdoor disconnect is off
Central AC systems have two parts: the indoor air handler or furnace, and the outdoor condenser. They’re usually on separate breakers. If only the indoor part has power, you’ll feel air from the vents but it won’t be cold.
Symptoms: the indoor fan runs, but the outdoor unit is silent.
What you can do:
- Reset a tripped breaker once by switching it fully OFF and then ON.
- Check that the outdoor disconnect switch is on.
Call a pro if: the breaker trips again. A breaker that trips repeatedly is telling you something is wrong, such as a shorted compressor, a failing fan motor or loose wiring. Don’t keep resetting it.
4. The outdoor unit is dirty or blocked
The outdoor condenser releases the heat taken out of your home. If its coil is caked with dirt, grass clippings, cottonwood fluff or dryer lint, or if shrubs crowd it, it can’t get rid of that heat, so your house stays warm and your bills go up.
Symptoms: the system runs constantly but can’t keep up on hot afternoons, and the air from the top of the outdoor unit feels extremely hot.
What you can do:
- Clear plants, leaves and debris at least 2 feet around the unit, and 5 feet above it.
- With the power off at the disconnect, gently rinse the coil fins from the outside in with a garden hose on a soft spray. Never use a pressure washer, which flattens the delicate fins.
- Our step-by-step guide on how to clean AC condenser coils covers this in detail.
5. The capacitor has failed
The capacitor gives the compressor and outdoor fan motor the jolt they need to start. It’s one of the most common parts to fail, especially after heat waves, and it’s a relatively inexpensive repair.
Symptoms: the outdoor unit hums or clicks but the fan doesn’t spin, the fan spins slowly, or the unit tries to start and then stops. A visibly bulging or leaking capacitor is a giveaway.
What you can do: turn the system off at the thermostat to prevent the compressor from overheating while you wait for service.
Call a pro: capacitors store a high-voltage charge even after the power is turned off and can cause a serious shock. Replacement is quick for a technician and usually costs $150 to $450 including the service call. See bad AC capacitor symptoms for more.
6. The evaporator coil is frozen
When the indoor coil gets too cold, moisture freezes on it and blocks airflow completely. Ice is a symptom, not the root cause. The usual causes are low airflow (dirty filter, closed vents, failing blower) or low refrigerant.
Symptoms: little or no air from the vents, ice or frost on the copper lines near the outdoor unit or on the indoor coil, water around the indoor unit as it melts.
What you can do:
- Turn the thermostat to OFF and set the fan to ON. The blower will melt the ice faster. This can take 2 to 24 hours.
- Replace the air filter and open every supply vent.
- Once the ice is fully gone, switch back to COOL.
Call a pro if: it freezes again within a day or two. That usually points to a refrigerant leak or a blower problem. Our guide to a frozen AC evaporator coil explains the causes in depth.
7. The system is low on refrigerant (there’s a leak)
An air conditioner doesn’t “use up” refrigerant. It circulates in a sealed loop. If the charge is low, there’s a leak somewhere, and topping it off without finding the leak only buys time.
Symptoms: air from the vents is cool but not cold, a hissing or bubbling sound near the indoor or outdoor unit, ice on the refrigerant lines, higher electric bills.
What you can do: nothing safely, beyond turning the system off to protect the compressor.
Call a pro: handling refrigerant requires EPA Section 608 certification. Ask the technician to find and repair the leak, not just recharge the system. If your unit uses R-22, which is no longer produced in the U.S., recharging is very expensive and replacement often makes more sense. Older R-410A systems are also affected by the industry’s move to newer refrigerants. See our R-410A phase-out guide.
8. The condensate drain is clogged
Your AC removes humidity, and that water drains away through a condensate line. Algae and sludge can clog it. Many systems have a float safety switch that shuts the AC off when the drain pan fills, to prevent water damage.
Symptoms: the AC suddenly stops, water in the drain pan or around the indoor unit, a musty smell.
What you can do:
- Find the drain line (usually a white PVC pipe near the indoor unit) and the access cap or T-fitting.
- Use a wet/dry vacuum on the outdoor end of the line for 1 to 2 minutes to pull out the clog.
- Pour a cup of distilled white vinegar into the access port to discourage future buildup. Repeat every few months during cooling season.
Our guide on how to unclog an AC condensate line shows each step.
9. The outdoor fan motor or contactor has failed
The contactor is an electrical switch that tells the outdoor unit to run. The fan motor moves air across the condenser coil. If either fails, the compressor overheats or never starts.
Symptoms: the outdoor fan doesn’t spin while the compressor hums, the fan spins backward or slowly, or the unit keeps running when the thermostat is off (a stuck contactor).
What you can do: turn the system off at the thermostat. A stuck contactor can run the compressor nonstop and damage it.
Call a pro: both repairs involve high-voltage components. A contactor typically costs $150 to $400 installed, and a condenser fan motor $300 to $700. See outdoor AC fan not spinning.
10. Leaky or poorly designed ductwork
Ducts that run through hot attics or crawl spaces can lose a large share of cooled air through leaks and poor insulation. Disconnected or crushed ducts can starve whole rooms.
Symptoms: some rooms are much warmer than others, upstairs is always hot, the attic is unusually cool, dusty air.
What you can do:
- Check that all supply registers and any manual balancing dampers on the ducts are open.
- If ducts in the attic or basement are accessible, look for disconnected sections and obvious gaps.
Call a pro: for a duct leakage test and sealing. Sealing with mastic and adding insulation can noticeably improve comfort. For two-story homes, see our guide on why upstairs is hot when the AC is running.
11. The system is the wrong size
An undersized AC runs nonstop on hot days and still can’t keep up. An oversized one cools the air quickly, shuts off, and leaves the house cold and clammy because it never runs long enough to remove humidity. Both problems are common after additions, remodels or a replacement sized by “rule of thumb.”
Symptoms: undersized: runs constantly, can’t reach the setpoint above roughly 90°F outside. Oversized: short, frequent cycles and high indoor humidity.
What you can do: compare your system’s tonnage (from the model number on the outdoor unit) with your home’s size using our AC size and BTU chart. It’s a rough check, not a substitute for a professional load calculation.
Call a pro: a proper sizing uses a Manual J load calculation, which accounts for insulation, windows, orientation and climate. Ask for one before replacing any system.
12. The compressor is failing
The compressor is the heart of the system and the most expensive part to replace. Compressor failure is often the end result of other problems left unfixed: low refrigerant, a bad capacitor, or a dirty condenser.
Symptoms: the outdoor fan runs but the air is warm, loud clanking or grinding from the outdoor unit, breakers tripping, or the unit won’t start at all.
Call a pro: a technician can test the compressor and its start components. A replacement compressor often costs $1,500 to $3,000 or more. On a system older than 10 to 12 years, replacing the whole system is often the better investment. Our guide to the repair vs. replace decision walks through the math.
When to call an HVAC technician
Call a licensed professional if:
- A breaker trips more than once.
- You see ice that returns after thawing, or you hear hissing near refrigerant lines.
- The outdoor unit hums but its fan doesn’t spin.
- You smell burning or see scorch marks.
- The problem involves refrigerant, the electrical compartment, or the compressor.
- You’ve done all the quick checks and the system still doesn’t cool.
If your AC is more than 10 to 15 years old and needs a major repair, get a replacement quote too. See our central AC replacement cost guide to know what a fair price looks like.
How to prevent AC cooling problems
- Change the air filter every 1 to 3 months.
- Rinse the outdoor coil each spring and keep 2 feet of clearance around the unit.
- Flush the condensate line with vinegar a few times each cooling season.
- Schedule a professional tune-up once a year, ideally in spring before the first heat wave.
- Don’t set the thermostat extremely low to “cool faster.” It won’t, and it makes the system run longer.
Our seasonal HVAC maintenance checklist covers everything in one place.
Frequently asked questions
Why is my AC running but not cooling the house?
The most common reasons are a dirty air filter, the thermostat fan set to ON instead of AUTO, a tripped breaker for the outdoor unit, or a dirty outdoor coil. If those check out, a failed capacitor, a refrigerant leak or a frozen coil are the next most likely causes.
How long should it take an AC to cool a house?
A properly sized and working system typically lowers the temperature about 1 to 2°F per hour on a hot day, depending on insulation, sun exposure and how hot the house got. If the temperature isn’t dropping at all after an hour, something is wrong.
Should I turn off my AC if it’s not cooling?
Yes, if you see ice, hear unusual noises, or the outdoor unit hums without the fan spinning. Running a struggling system can damage the compressor, the most expensive part to replace.
Can I add refrigerant to my AC myself?
No. Refrigerant handling requires EPA Section 608 certification, and a low charge means there’s a leak that needs to be found and repaired.
How much does it cost to fix an AC that isn’t cooling?
Many fixes cost nothing (a thermostat setting or a breaker) or under $50 (a filter). Common professional repairs like a capacitor or contactor typically run $150 to $450. Refrigerant leak repairs and compressor replacements cost much more.
Disclaimer: this guide is for general information only. HVAC systems involve high voltage, refrigerant and, in some cases, gas. When in doubt, contact a licensed HVAC professional.