Why Is My AC Not Cooling? Troubleshooting Guide (Before You Call Anyone)
You hear the air conditioner running. The vents are blowing. But your house feels like an oven and the thermostat says it's 82 degrees when you set it to 72.
Before you panic and call for emergency service, you can solve many AC cooling problems yourself. Some issues take five minutes and zero tools to fix. Others need professional help, but knowing which is which saves you time and money.
This guide walks through the most common reasons your AC stops cooling properly. You'll learn what to check yourself, what's safe to try, and when it's time to call in the pros. Most importantly, you'll know if you're looking at a $20 filter or a $2,000 repair.
Start With the Thermostat (Seriously)
Check your thermostat settings first. This fixes the problem 30% of the time.
Make sure it's set to "cool" not "heat" or "off." Someone might have bumped it without realizing. Verify the temperature is set lower than your current room temperature. If it says 75 and your house is 74, the AC won't run.
Look at the fan setting. It should be on "auto" not "on." When set to "on," the fan runs constantly even when the AC isn't cooling. This blows warm air through your house and makes you think the system isn't working.
Try this quick reset: Turn the thermostat completely off. Wait two minutes. Set it back to cool with the temperature at least 5 degrees below room temperature. Listen for the AC to kick on within a few minutes.
If you have a programmable or smart thermostat, check the schedule. Maybe it's programmed for energy saving when you actually want cooling. Dead batteries in thermostats cause all kinds of weird behavior, so swap those out if your thermostat uses them.
Still not working? The thermostat itself might be broken, but keep checking the other issues below before assuming that's the problem.
Check Your Air Filter (The Most Common Culprit)
A clogged air filter causes 60% of AC cooling problems. This is the easiest fix and you should check it first after the thermostat.
Find your air filter. It's usually in the return air vent (the big grate on your wall or ceiling) or in a slot near your air handler in the basement or closet.
Pull it out and hold it up to light. If you can't see light through it clearly, it's too dirty. Even if it looks somewhat clean, replace it if you haven't changed it in over two months.
Here's what happens with a dirty filter: Airflow gets restricted. Less air moves across the evaporator coil. The coil can't absorb heat properly. Ice might form on the coil. Your house stays hot even though the AC runs constantly.
Replace the filter with the correct size. Check the dimensions printed on the old filter's frame. Install it with the arrow pointing toward the AC unit, not toward the room.
After replacing the filter, give the system 30 minutes to start cooling properly. If the coil was frozen, it needs time to thaw and drain.
Regular filter changes every 1 to 2 months prevent this problem entirely. Set a phone reminder so you don't forget. Consider joining a maintenance program that handles this automatically.
Look for Ice on Your AC Unit
Ice on your indoor or outdoor AC unit means something is seriously wrong. The system can't cool your house when covered in ice, even though it seems backwards.
Where to Look for Ice
Check your indoor air handler if you can access it. Look at the copper lines going into the unit. Ice coating these lines means problems.
Go outside and inspect your condenser unit (the big box with the fan). Ice or frost on the refrigerant lines or around the unit indicates airflow or refrigerant issues.
What Causes Ice Formation
The most common causes are:
Dirty air filter restricting airflow
Low refrigerant levels from leaks
Blocked return vents in your home
Dirty evaporator coil
Broken blower fan
What to Do About Ice
Turn the AC off immediately. Switch your thermostat to "off" or "fan only" mode. Let the ice melt completely. This takes 2 to 4 hours minimum.
Don't try to chip or scrape the ice off. You'll damage the coils. Just let it thaw naturally.
After the ice melts, replace your air filter if you haven't already. Check that all your supply and return vents are open and unblocked.
Turn the AC back on. If ice returns within a few hours, you have a refrigerant leak or mechanical problem. Time to call for professional AC repair.
Check All Your Vents and Registers
Blocked vents reduce cooling by up to 40% in affected rooms. Walk through your house and verify every vent.
Open all supply vents (the ones blowing cold air) completely. Furniture, curtains, or closed dampers block airflow. Even partially closed vents cause problems.
Make sure return vents (the ones sucking air in) are clear too. These need at least 6 inches of clearance. People put furniture right against returns without realizing it kills AC performance.
Check your basement or crawlspace if you have one. Disconnected or damaged ductwork down there means cold air never reaches your living space. Look for obvious gaps or sections that fell apart.
Close all windows and exterior doors. An open window defeats your AC faster than any mechanical problem. Sounds obvious but happens more than you'd think.
Seal gaps around window AC units if you have them. Those foam insulation strips deteriorate. Replace them when they get crumbly or compressed.
Inspect Your Outdoor Condenser Unit
Your outdoor unit needs airflow to work. Go outside and take a close look.
Clear debris from around the unit. Leaves, grass clippings, cottonwood fluff, and bushes block airflow. You need at least 2 feet of clearance on all sides.
Look at the fins on the sides of the unit. These thin metal pieces bend easily. Bent fins restrict airflow significantly. You can buy a fin comb at hardware stores to straighten them, but be gentle. Don't stick screwdrivers or other tools in there.
Check if the fan is spinning. With the AC running, the outdoor fan should turn. If it's not spinning but you hear humming, the fan motor likely failed. Shut the system off and call for repair before you damage the compressor.
Hose off the outside of the unit gently. Don't use a pressure washer. Regular water pressure from a garden hose works fine. Spray from the inside out to push dirt away from the fins, not into them.
Make sure the unit is level. If it's tilted or sinking into the ground, refrigerant can't flow properly. This requires professional correction.
Wondering whether to repair or replace your AC? Check out our guide on when to repair vs replace your HVAC system for the honest math on making this decision.
Circuit Breaker and Electrical Issues
AC units trip breakers more often than you'd expect. Check your electrical panel.
Find the breaker labeled for your AC or air conditioner. If it's in the middle position (tripped), flip it completely off first, then back on. Don't just push it to "on" from the middle position. That doesn't reset it properly.
Look for a disconnect box near your outdoor unit. It's a small metal or plastic box mounted on the wall near the condenser. Open it and check if the fuse or breaker inside is intact. Replace blown fuses with identical amp ratings.
If the breaker trips repeatedly, stop resetting it. Multiple trips indicate a serious electrical problem or failing compressor. Continuing to reset causes equipment damage and fire risk.
Verify power to your indoor air handler too. Some systems have separate breakers for indoor and outdoor units. Both need power to cool properly.
Loose electrical connections cause intermittent cooling problems. If your AC works sometimes but not others, electrical issues are likely. This requires professional diagnosis.
Refrigerant Leaks (You Can't Fix This Yourself)
Low refrigerant is the second most common cooling problem after dirty filters. You can't add refrigerant yourself. It requires EPA certification and special equipment.
Signs of Low Refrigerant
Watch for these indicators:
Ice on refrigerant lines or coils
AC runs constantly but doesn't cool well
Hissing or bubbling sounds near the unit
Higher than normal electric bills
Warm air from vents even when AC runs
Why Refrigerant Gets Low
Air conditioners don't consume refrigerant like gas in a car. If it's low, you have a leak. Simply adding more refrigerant without fixing the leak wastes money and damages the environment.
Leaks happen in refrigerant lines, coil connections, the evaporator coil itself, or the condenser coil. Finding leaks requires specialized detection equipment.
Refrigerant refills cost $200 to $800 depending on system size and refrigerant type. Older R-22 refrigerant (Freon) costs significantly more than newer R-410A. If you have an old system using R-22, consider whether AC replacement makes more financial sense than expensive refills.
The leak repair itself adds to the cost. Small leaks in accessible locations run $200 to $500. Leaks inside coils often mean replacing the entire coil for $1,000 to $2,500.
Compressor Problems (The Expensive One)
The compressor is your AC's heart. When it fails, you're looking at major repair or replacement decisions.
Listen for unusual sounds from your outdoor unit. A failing compressor makes hard starting sounds (struggling to start), loud humming, or clicking without the fan running.
Check if the outdoor unit vibrates excessively. Some vibration is normal, but severe shaking indicates compressor or mounting problems.
Compressor failure usually means replacement, not repair. Compressor replacement costs $1,500 to $2,500 installed. On systems over 10 years old, this repair rarely makes sense financially.
Apply the same decision framework from our furnace repair vs replacement guide. If the repair costs more than 50% of a new system and your AC is past half its expected 15 year lifespan, replacement wins.
Compressor warranties vary by manufacturer. Some offer 10 year coverage on the compressor itself, though you still pay labor costs. Check your system documentation or contact the best HVAC companies in your area for warranty verification.
When to Call for Professional Help
Stop DIY troubleshooting and call a professional if:
You smell burning or unusual chemical odors. Electrical problems or refrigerant leaks cause these smells and create safety hazards.
You hear loud grinding, squealing, or banging noises. These indicate mechanical failure that worsens with continued operation.
Water leaks inside your home near the air handler. Clogged condensate drains or broken drain pans need immediate attention to prevent water damage.
The AC won't turn on at all after you've checked the breaker and thermostat. Complete system failure requires diagnostic equipment.
Ice keeps forming even after you've replaced filters and cleared vents. Refrigerant or mechanical issues need professional repair.
Your AC is over 12 years old and needs major repairs. At this age, evaluate replacement vs repair carefully with professional guidance.
You've tried everything in this guide and the system still won't cool properly. Some problems require specialized tools, gauges, and knowledge.
Find qualified help through professional AC repair services that offer transparent pricing and honest assessments. Ask about diagnostic fees upfront. Most companies charge $75 to $150 for diagnosis, credited toward repair if you proceed.
Preventive Maintenance to Avoid Future Problems
Annual AC maintenance prevents 80% of cooling failures. Professional tune ups catch small problems before they become expensive emergencies.
What Professional Maintenance Includes
A complete AC tune up covers:
Refrigerant level check and adjustment
Electrical connection inspection and tightening
Condenser and evaporator coil cleaning
Condensate drain cleaning
Blower motor lubrication
Thermostat calibration
Airflow measurement and optimization
Professional maintenance costs $100 to $200 annually for most systems. Maintenance programs often include priority scheduling and repair discounts that offset the cost.
DIY Maintenance You Can Do
Between professional visits, handle these tasks:
Change filters monthly during cooling season
Keep outdoor unit clear of debris
Trim vegetation 2 feet away from condenser
Clean supply and return vents
Check thermostat batteries twice yearly
Schedule professional maintenance in spring before cooling season starts. This catches problems when technicians aren't overwhelmed with emergency calls and you have time to make informed repair decisions.
Frequently Asked Questions
Why is my AC running but not cooling?
The most common cause is a dirty air filter restricting airflow. Other frequent culprits include low refrigerant levels, frozen evaporator coils, blocked condenser units, or thermostat problems. Check your filter first, then inspect for ice on the indoor coils or outdoor unit. If those aren't the issue, you likely need professional AC diagnosis.
How long should it take my AC to cool my house?
Your AC should cool your home 1 degree every 15 to 20 minutes under normal conditions. On extremely hot days (95°F+), cooling slows to about 1 degree per 30 minutes. If your house isn't cooling at least 1 degree per hour, something is wrong. Check your filter, verify all vents are open, and make sure the outdoor unit has clear airflow.
Can I run my AC with a dirty filter?
No, running your AC with a dirty filter causes expensive damage. Restricted airflow makes the evaporator coil freeze, strains the blower motor, and reduces cooling efficiency by up to 15%. The blower motor working harder shortens its lifespan and increases your electric bill. Change filters every 1 to 2 months during cooling season to prevent these problems.
Why does my AC freeze up?
AC units freeze when airflow is restricted or refrigerant levels are low. Dirty filters cause 70% of freeze ups by blocking air across the evaporator coil. Low refrigerant from leaks causes the remaining 30%. Turn the system off immediately when you see ice, let it thaw completely (2 to 4 hours), replace the filter, then restart. If ice returns, you have a refrigerant leak requiring professional repair.
How much does AC repair cost?
Basic AC repairs cost $150 to $500 for common issues. Refrigerant refills run $200 to $800, compressor replacements cost $1,500 to $2,500, and evaporator coil replacements range from $1,000 to $2,000. Simple fixes like capacitor replacement ($150 to $400) or contactor replacement ($100 to $300) are much cheaper. Get written estimates before authorizing major repairs. Compare repair costs to AC replacement pricing if your system is over 12 years old.
When should I replace my AC instead of repairing it?
Replace your AC if it's over 12 years old and needs repairs costing more than $1,000. Also replace if you've spent over $1,500 on repairs in the past two years, if the compressor failed, or if it uses R-22 refrigerant (Freon) which is being phased out. New AC systems are 30% to 50% more efficient than 12 year old units, so replacement often pays for itself through energy savings over 5 to 7 years.
What temperature should I set my AC to?
Set your AC to 78°F when you're home for the best balance of comfort and efficiency. Every degree lower increases your cooling costs by 3% to 5%. At night, 72°F to 75°F works well for sleeping. When you're away, 82°F to 85°F prevents excessive humidity while saving energy. Programmable thermostats automate these settings so you don't have to remember.
Still can't get your AC cooling properly? Contact experienced HVAC professionals for honest diagnostics and transparent pricing. The right technician explains what's wrong, gives you options, and helps you make informed decisions about repair or replacement.