Why your AC isn't cooling: The most common causes and fixes

• Common reasons an AC runs but doesn't cool include a clogged filter, incorrect thermostat settings, or a dirty outdoor unit.
• Professional help is needed for issues like refrigerant leaks, ice on the coils, or a failing compressor.
• Regular maintenance, such as changing filters and scheduling annual tune-ups, can prevent many common AC problems.

Nothing ruins a perfect summer day like a broken air conditioner. You come inside expecting to be greeted by a blast of cool air, but are met with stale, lukewarm air instead.  

When your AC system isn’t working, your home quickly becomes so unbearable that you might as well head back outside. At least out there you can find fresh air and a breeze or two. 

Fortunately, a lack of cold air doesn’t always mean you need a brand-new HVAC system. But it certainly means you need a professional HVAC company to come to your home as soon as possible. 

Here, USA TODAY breaks down common reasons why your AC is not cooling but running. We explore simple AC troubleshooting tips, DIY fixes and how to know when it’s time to call in a professional. 

Why your AC isn’t cooling and what to do about it 

An air conditioner blowing warm air is a surprisingly common issue for homeowners. Often, the system is still running and blowing air through the vents, but the temperature never drops.  

When central air is not cooling the house properly, the culprit usually falls into one of two categories:  

• A simple airflow restriction you can fix yourself  
• A mechanical failure that requires an expert 

Before calling for an appointment, running through a basic checklist may help you diagnose the problem yourself. Plus, understanding how your unit functions will make it easier to pinpoint the exact failure. 

How your AC is supposed to work 

An air conditioner doesn’t actually create cold air. It works by removing heat and humidity from your indoor spaces and dumping it outside. But to achieve that relocation, the process must work in a continuous loop: 

• An indoor fan pulls warm air across a cold evaporator coil. 
• Chemical refrigerant inside the coil absorbs the heat from your indoor air. 
• The cooled air is pushed back through your ductwork into your home. 
• The refrigerant travels to the outdoor condenser unit, where a compressor releases the trapped heat outside.

“The importance of maintenance may change a little depending on where you are in the U.S., but biannual maintenance is the baseline,” says Joey Dooley, a general manager at ARS in Texas with more than 20 years of experience in the HVAC industry. “Almost every manufacturer recommends it, and I can tell you that a lot of them also effectively require maintenance if you want to protect your warranty.” 

Protecting your HVAC warranty is one thing, but regular maintenance also protects your wallet. The U.S. Department of Energy insists that regular HVAC maintenance is vital to keeping the heat-exchange process working efficiently. If any part of this cycle gets interrupted, your system will struggle to lower the temperature and likely require repairs. 

Most common reasons your AC isn’t cooling 

Some most common reasons your AC isn’t cooling can be fixed without help from a pro. These include: 

• Clogged air filter: A dirty filter can block airflow, preventing warm air from passing over the cooling coils. Failing to replace your filter when needed can lead to an inefficient AC system. “[Modern] systems can help you understand what’s really going on and help diagnose problems before you get a panic call or a major failure,” Dooley says. In many cases, this issue is avoidable. 
• Incorrect thermostat settings: Sometimes the fix is incredibly simple. Check your thermostat to see if it was accidentally set to “Heat,” or if the fan is set to “On” instead of “Auto.” Setting your thermostat to “Auto” is a good way to save energy and ensure your AC responds accurately to the indoor temperature. 
• Dirty outdoor condenser unit: Your outdoor unit needs space to release heat. If it’s choked by tall grass, leaves or dirt, the heat gets trapped inside the system. Clean the surrounding area to ensure at least two feet of clear space on all sides. 
• Low or leaking refrigerant: Refrigerant is the lifeblood of the cooling process. If your system develops a small leak, it won't have enough chemical power to absorb indoor heat. This typically causes the system to blow lukewarm air. 

Quick AC troubleshooting checklist 

Before spending money on a service call, run through these quick steps to rule out minor issues: 

• Check the thermostat: Ensure it’s set to “Cool,” and the temperature is lower than the current room temperature. In some cases, smart home technology can diagnose a problem for you and report it on your home’s thermostat hub. 
• Inspect the air filter: Swap out your filter if it looks grey or is caked in dust. 
• Look at the outdoor unit: Clear away any debris, branches or weeds blocking the metal fins. 
• Check your circuit breakers: Make sure the breakers for both the indoor blower and outdoor unit are in the “On” position. 

When to call an HVAC professional to fix your AC 

While homeowners can handle filters and basic cleaning, certain tasks are dangerous or require specialized tools.  

For instance, Dooley notes that one of the most common early issues HVAC technicians see is a bad capacitor. “Depending on where you live in the country, it really feels like most people will have at least one failure within the first year or two of the system. Capacitors just don’t last as long as they used to.” 

Diagnosing a capacitor issue at home is tricky. So, once you’ve gone through the troubleshooting checklist, it may be time to call a certified technician. You should call a technician immediately if you notice: 

• Hissing or bubbling noises coming from the refrigerant lines. 
• Ice forming on the copper lines or indoor evaporator coil. 
• The outdoor compressor clicks repeatedly but fails to turn on. 
• Electrical burning smells or smoke coming from any part of the system. 

Handling chemical refrigerants can be dangerous, and you often need EPA certification to purchase them. Let the HVAC pros fix AC refrigerant-related issues, don’t risk DIY-ing it. 

How much it costs to fix your AC 

AC repair costs vary depending on what has failed. Minor adjustments might only cost a standard diagnostic fee, while major part replacements require more significant investments. 

“The range can be anywhere from $8,000 to $35,000,” says Josh Condor, Division Operations Manager for ARS Central Division, a national HVAC installer. “I know it’s good to have a solid answer, but there are so many variables and factors that play into that: the size of their ductwork, the current situation of their home...”  

The size of your home, system efficiency, AC brand, ductwork and more can play into how much it costs to get a new air conditioner. 

How to prevent AC cooling problems 

The best way to deal with an AC not cooling is to prevent problems from happening in the first place. Regular maintenance is crucial to keeping your home comfortable. 

These simple habits can help keep your AC running smoothly: 

• Change filters regularly: Replace standard filters every 30 to 90 days. 
• Schedule annual tune-ups: Have a technician inspect your system every spring before the summer heat hits. 
• Keep registers clear: Ensure furniture and curtains aren't blocking your indoor air supply vents. 

Investing a small amount of time into preventative maintenance could extend your system's lifespan and lower your monthly energy bills. 

Frequently asked questions 

Why is my AC running but not cooling? 

This usually indicates an airflow restriction, like a clogged filter, or a mechanical issue like a failing capacitor or low refrigerant. If you can't resolve the issue yourself by cleaning a filter or adjusting the thermostat, call a certified technician to get a more precise diagnosis. 

How do I reset my AC? 

Each brand and model may be a little different, but turning off the unit, waiting 60 seconds, and then turning it back on could help. You could also turn off the AC circuit breaker on your electrical panel and give it a minute before turning it back on. 

Why is my AC blowing warm air suddenly? 

A sudden blast of warm air could mean the outdoor unit has lost power, or the compressor has overheated. Check the circuit breaker to see if it tripped. 

Can low refrigerant fix itself? 

No. Air conditioners operate on a closed loop and don’t consume refrigerant like gas in a car tank. If the refrigerant is low, you have a physical leak that must be repaired by a professional.