Why Is My AC Not Cooling? 12 Common Problems and Quick Fixes

Last night at 2 AM, I got the call every HVAC tech knows too well: "My AC is running but it's 85 degrees in here!" After walking the homeowner through checking their filter (completely clogged), their AC was cooling again in 10 minutes. That's the thing about air conditioners - when they stop cooling, it's often something simple you can fix yourself. Let me share what causes 90% of the "AC not cooling" calls I get, starting with the fixes you can do right now.

1. Dirty or Clogged Air Filter (Fixes 30% of Cases)

I can't tell you how many $150 service calls end with me holding up a filter that looks like a carpet sample. When your filter gets this dirty, airflow drops to almost nothing. Your evaporator coil freezes into a block of ice, and suddenly you're getting no cooling at all. The crazy part? Homeowners often say "but I just changed it!" - turns out "just" was six months ago.

2. Thermostat Settings Wrong (The Embarrassing One)

You'd be surprised how often this happens. Just last week, a customer swore their AC was broken. Turns out their kid had switched it to "heat" mode while playing with the buttons. I've also seen programmable thermostats set to "away" mode or with the fan set to "on" instead of "auto," making people think they're getting warm air when it's just the fan running between cooling cycles.

Check these settings:

• Mode: Set to "Cool" (not Heat, Fan, or Off)
• Temperature: Set below current room temperature
• Fan: Set to "Auto" (not On)
• Schedule: Disable if you're using a programmable model
• Batteries: Replace if display is dim or blank

3. Frozen Evaporator Coils (The Ice Block Problem)

Open your indoor unit's panel and you might find something that looks like an ice sculpture convention. When evaporator coils freeze, your AC becomes a very expensive fan. This happens when airflow is restricted (dirty filter, closed vents) or refrigerant is low. I once pulled out a solid 50-pound block of ice from a unit - the homeowner couldn't believe all that ice was blocking their cooling.

4. Blocked or Closed Vents (The Hidden Airflow Killer)

Here's what drives me crazy: people close vents thinking they'll save money, but it actually makes their AC work harder and cool worse. I visited one home where 8 out of 12 vents were closed or blocked by furniture. The system was struggling to push air through the few open vents, causing the evaporator to freeze. Your AC needs all vents open to work properly - it's designed for a specific airflow.

5. Dirty Outdoor Unit (Condenser Coils)

Your outdoor unit can't reject heat when it's covered in grass clippings, cottonwood seeds, or dog hair. I've seen units so clogged they looked like Chia Pets. When the condenser can't breathe, your system can't cool. One customer's AC started working perfectly after we removed three years of dryer lint that had been blowing onto it.

6. Refrigerant Leak (The Expensive One)

If your AC is low on refrigerant, it's leaking somewhere - refrigerant doesn't just disappear. Signs include ice on the copper lines, hissing sounds, or the AC running constantly without cooling. Last month, I found a leak in a 3-year-old system caused by a lawn mower throwing a rock into the coil. The homeowner had been adding refrigerant yearly instead of fixing the leak - that's like putting air in a flat tire every day instead of patching it.

7. Failed Capacitor (The $200 Fix)

Capacitors are like batteries that give your compressor and fan motors the jolt they need to start. In the heat of summer, they fail constantly. Your AC might hum but not start, or the outdoor fan won't spin. I keep a dozen capacitors in my truck because I replace at least two every hot week. Good news: it's a quick, relatively cheap fix that gets you cooling again fast.

8. Tripped Breaker or Blown Fuse

Sometimes your AC stops cooling because it's not getting power. Check your electrical panel for any tripped breakers (they'll be in the middle position, not fully "on" or "off"). Your outdoor unit might also have a disconnect box with fuses. I once spent 20 minutes troubleshooting before realizing the homeowner's teenager had tripped the breaker while resetting WiFi equipment in the garage.

9. Compressor Problems (The Heart Attack)

The compressor is your AC's heart - when it fails, you're looking at major surgery or replacement. You'll know it's the compressor if the outdoor fan runs but you don't hear the compressor's distinct hum, or if it makes clicking sounds trying to start. Compressors usually fail from old age, power surges, or running with low refrigerant for too long. This isn't a DIY fix - and honestly, if your compressor dies on an older unit, it's often cheaper to replace the whole system.

10. Clogged Condensate Drain Line

Your AC removes gallons of water from your home's air daily. When the drain line clogs with algae or gunk, water backs up and triggers a safety switch that shuts everything down. I've pulled some nasty stuff out of drain lines - one had a gecko that decided to explore and got stuck. Pour a cup of vinegar down the drain monthly to prevent clogs.

11. Leaky or Poorly Insulated Ductwork

You could be cooling your attic instead of your house. I crawled into an attic last summer where the duct had completely separated - the AC was blasting cold air straight into the 140-degree attic. The homeowners couldn't figure out why their electric bill tripled. Even small leaks waste tons of cooled air. If certain rooms won't cool or you feel air in your attic, you've got duct problems.

12. Wrong Size AC Unit (The Permanent Problem)

Here's the truth nobody wants to hear: if your AC never cooled well, it might be the wrong size. An undersized unit runs constantly but can't keep up. An oversized unit cycles on and off rapidly, never removing humidity, leaving you cool but clammy. I see this constantly in additions where they just extended the existing system. Proper sizing requires load calculations - not just guessing based on square footage.

Prevention: Stop Problems Before They Start

After 20 years in HVAC, here's what actually prevents these problems: Change your filter monthly during heavy use (set a phone reminder). Keep 2 feet clear around your outdoor unit. Pour vinegar down your condensate drain quarterly. Have a pro check refrigerant levels and electrical components yearly - ideally in spring before you need cooling. These simple steps prevent 80% of emergency calls.

The Bottom Line: Start Simple

When your AC stops cooling, don't panic. Check your filter first - seriously, it's the problem more often than you'd think. Then verify your thermostat settings, look for ice, and make sure all vents are open. These simple checks solve most cooling problems and cost you nothing but time. If these don't work, that's when you check the outdoor unit, breakers, and drain line.

Remember, your AC doesn't just randomly break - something causes it to fail. Regular maintenance catches small problems before they become midnight emergencies. And if you do need to call a pro, knowing what you've already checked helps us diagnose faster and saves you money on the service call.