Quick Safety Note
Turn the AC off at the breaker if you see ice on refrigerant lines, smell burning, or hear grinding. Running a malfunctioning AC can destroy the compressor ($1,800+ repair).
Last night at 2 AM I got the call every HVAC tech knows: "My AC is running but it's 85°F in here." After walking the homeowner through checking the filter (completely clogged), the AC was cooling again in 10 minutes. That is the thing about ACs. When they stop cooling, it is usually something simple you can fix yourself.
Here are the 12 reasons behind almost every "AC not cooling" call I take, starting with the easiest DIY fixes.
1. Dirty or Clogged Air Filter (Fixes 30% of Cases)
I cannot count how many $150 service calls end with me holding up a filter that looks like a carpet sample. When the filter gets that dirty, airflow drops to almost nothing, the evaporator coil freezes into a block of ice, and suddenly you get no cooling at all. Homeowners often say "but I just changed it" and turns out "just" was six months ago.
Find the return vent (usually the biggest grille in your home). Pull the filter and hold it up to a light. If light does not pass through, it is too dirty. Replace it, write the date on the new one, and set a phone reminder for 30 days. If you already see ice on the indoor unit, turn cooling off but leave the fan on for 2 to 4 hours to defrost before restarting.
2. Thermostat Settings Wrong (The Embarrassing One)
You would be surprised how often this is the answer. 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 have also seen programmable thermostats stuck in Away mode or with the fan set to On instead of Auto, which keeps the fan blowing room-temperature air between cooling cycles and feels like warm air.
Check the basics: Mode set to Cool, temperature setpoint below current room temp, Fan set to Auto (not On), schedule disabled if you are testing, and fresh batteries if the display is dim. Five seconds of checking saves a service call.
3. Frozen Evaporator Coil (The Ice Block)
Open the indoor unit panel and you might find an ice sculpture. When the evaporator coil freezes, your AC becomes a very expensive fan. Causes are restricted airflow (dirty filter, closed vents) or low refrigerant. I once pulled out a 50-pound block of solid ice. The homeowner could not believe all that ice was blocking their cooling.
To defrost: turn the thermostat to Off (cooling off), set the fan to On, replace the filter if dirty, and wait 3 to 4 hours with towels ready for melting water. Once defrosted, switch back to Cool. If the coil freezes again within 48 hours, you have a refrigerant leak. Call a pro.
4. Blocked or Closed Vents
Here is what drives me crazy: people close vents thinking they will save money, but it makes the AC work harder and cool worse. I visited one home where 8 of 12 vents were closed or blocked by furniture. The system was struggling to push air through the few open vents and the evaporator iced over. Your AC needs nearly all vents open to hit the designed airflow. Open every vent, move furniture off floor grilles.
5. Dirty Outdoor Condenser
Your outdoor unit cannot reject heat when it is wrapped in grass clippings, cottonwood seeds, or dog hair. I have seen units so clogged they looked like Chia Pets. One customer's AC started working perfectly after we removed three years of dryer lint that had been blowing onto it.
Safe cleaning: turn power off at the breaker, remove large debris by hand, spray the coils gently with a garden hose (NOT a pressure washer, which bends fins), spray from inside out to push debris away, clear 2 feet around the unit, let dry, restore power.
6. Refrigerant Leak (The Expensive One)
If your AC is low on refrigerant, it is leaking somewhere. Refrigerant does not just disappear. Signs: ice on the copper lines, hissing sounds, 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 "topping off" refrigerant yearly instead of fixing the leak. That is like adding air to a flat tire every day instead of patching it.
Modern equipment uses R-454B, which costs about three times more than the older R-410A refrigerant. A refill that used to cost $250 is now $400 to $600. Find and fix the leak rather than refilling annually.
7. Failed Capacitor (The $300 Fix)
Capacitors give your compressor and fan motors the jolt they need to start. In summer heat, they fail constantly. Your AC might hum but not start, or the outdoor fan refuses to spin. I keep a dozen capacitors in my truck because I replace at least two every hot week. Good news: it is quick, relatively cheap, and gets you cooling again fast. Bad news: do not DIY this. High voltage capacitors can discharge and shock you even when power is off.
8. Tripped Breaker or Blown Fuse
Sometimes the AC stops cooling because it is not getting power. Check your electrical panel for tripped breakers (they sit in the middle position, not fully On or Off). The outdoor unit has a disconnect box with fuses, open that too. I once spent 20 minutes troubleshooting before realizing the homeowner's teenager had tripped the breaker while resetting WiFi gear in the garage.
If the breaker trips again immediately after reset, do not keep resetting. Something is shorted and you risk a fire. Call a pro.
9. Compressor Problems (The Heart Attack)
The compressor is your AC's heart. When it fails, you are looking at major surgery or replacement. Signs: outdoor fan runs but you do not hear the compressor's distinct hum, or you hear clicking sounds as it tries to start. Compressors usually die from old age, power surges, or running for years with low refrigerant.
Not a DIY fix. Honestly, if your compressor dies on an older unit, replacing the whole system is often cheaper than the compressor repair, and you get warranty coverage on the new equipment.
10. Clogged Condensate Drain Line
Your AC pulls gallons of water out of your home's air daily. When the drain line clogs with algae or gunk, water backs up and trips a safety switch that shuts everything down. I have pulled some unpleasant things out of drain lines. One had a gecko that decided to explore and got stuck.
How to clear it: find the PVC drain line near the indoor unit, locate the T-fitting with a cap (access port), pour 1 cup of distilled white vinegar in, wait 30 minutes, flush with water. For stubborn clogs, hold a wet/dry vacuum to the outdoor end of the drain to suck the blockage out. Pour vinegar monthly to prevent algae buildup.
11. Leaky or Disconnected Ductwork
You could be cooling your attic instead of your house. I crawled an attic last summer where the supply duct had completely separated. The AC was blasting cold air straight into 140°F attic space. The homeowners could not figure out why their bill tripled. Even small leaks waste tons of conditioned air.
Signs of duct leaks: certain rooms never cool, conditioned air felt in attic or crawlspace, dust building up around supply registers, bills climbing without an obvious cause. Inspect the ductwork or hire a duct test ($150 to $400) to confirm.
12. Wrong-Sized AC Unit (The Permanent Problem)
The truth nobody wants to hear: if your AC never cooled well, it might be the wrong size. An undersized unit runs constantly and never reaches setpoint. An oversized unit cycles on and off rapidly, never pulls humidity, and leaves you cool but clammy. I see this constantly in additions where the contractor just extended the existing system without recalculating the load.
Proper sizing needs a Manual J load calculation, not square-footage guessing. The AC tonnage calculator gives a fast ballpark in two minutes if you want to compare against what is installed today. If your AC has always struggled and none of the fixes above help, sizing is likely the issue.
Call a Pro Immediately If You See
- Burning smell or visible smoke
- Electrical sparking or exposed wires
- Refrigerant leak (oil spots on copper lines, hissing sounds)
- Compressor making loud grinding or screeching
- Water leaking through a ceiling below an attic unit
- Breaker trips repeatedly after reset
Typical AC Repair Costs
What you should expect to pay for common AC repairs in most US markets:
| Repair | Typical Cost |
|---|---|
| Filter replacement (DIY) | $15 to $40 |
| Condensate drain clearing | $100 to $250 |
| Capacitor replacement | $200 to $400 |
| Refrigerant recharge (R-454B) | $400 to $700 |
| Refrigerant leak repair | $500 to $2,000 |
| Evaporator coil replacement | $1,200 to $2,500 |
| Compressor replacement | $1,800 to $3,500 |
| New AC system installed | $7,500 to $13,000 |
Refrigerant work climbed about 30% since the R-454B transition. If your compressor fails on a system 12+ years old, replacing the whole system is often the better spend. Use our HVAC installation cost calculator to compare.
Prevention: Stop Problems Before They Start
After 20 years in HVAC, here is what actually prevents these calls. Change your filter monthly during heavy use (set a phone reminder). Keep 2 feet clear around your outdoor unit. Pour 1 cup of distilled white vinegar down the condensate drain quarterly. Have a pro check refrigerant levels and electrical components annually, ideally in spring before you need cooling. These four habits prevent about 80% of the emergency calls I take.
Bottom Line: Start Simple
When your AC stops cooling, do not panic. Check the filter first. It is the problem more often than you would think. Then verify thermostat settings, look for ice, open all vents. These cost you nothing but time and fix the majority of cases. If those fail, check the outdoor unit, breakers, and drain line. Only after all of that should you call a pro. Knowing what you have already ruled out helps the tech diagnose faster and saves you money on the service call.