There's nothing more frustrating than an AC that runs but doesn't cool. Before you call a technician (and pay a service call fee), there are several things you can check yourself. This guide walks you through the most common causes of AC cooling problems, from simple DIY fixes to issues that require a professional.
Check These First (DIY Troubleshooting)
Before calling a professional, check these common and easily fixable issues:
1. Check your air filter. A clogged filter is the #1 cause of AC cooling problems. Dirty filters restrict airflow, which can cause the evaporator coil to freeze and stop cooling. Check your filter — if it's dirty, replace it. If the coil has frozen, turn off the AC and let it thaw for a few hours before restarting with a clean filter.
2. Check the thermostat. Make sure it's set to "cool" and the temperature is set below the current room temperature. If the display is blank, the batteries may need replacing. If it's a programmable thermostat, check that the schedule hasn't been accidentally changed.
3. Check the outdoor unit. Make sure the outdoor condenser unit is running. If it's not, check the circuit breaker — AC units draw significant power and can trip breakers. Also make sure nothing is blocking airflow around the unit (plants, furniture, debris). Keep at least 2 feet of clearance on all sides.
4. Check all vents. Make sure supply and return vents are open and unobstructed. Closing vents in unused rooms can actually reduce cooling efficiency by creating pressure imbalances.
Low Refrigerant (Needs a Pro)
If the filter is clean and the unit runs but the air coming from the vents isn't cold, low refrigerant is a likely cause. Refrigerant doesn't get "used up" — if your system is low, there's a leak somewhere.
Signs of low refrigerant:
- Air from vents is cool but not cold
- Ice on the refrigerant lines or evaporator coil
- Hissing or bubbling sounds (indicates a leak)
- Higher than normal electric bills
- AC runs constantly but can't reach the set temperature
Why you need a professional: Refrigerant handling requires EPA Section 608 certification — it's illegal for homeowners to add refrigerant themselves. A professional will find and repair the leak, then recharge the system. Cost: $200–$600 for a recharge, $400–$1,200 if leak repair is needed.
Frozen Evaporator Coil
If you see ice on the indoor unit or the refrigerant lines, the evaporator coil has frozen. This stops cooling and can damage the compressor if the AC keeps running.
Common causes:
- Dirty air filter (most common — replace it)
- Low refrigerant (needs a pro)
- Blocked airflow (closed vents, dirty ducts)
- Blower motor or fan issues
What to do: Turn off the AC immediately. Let the ice melt completely (this can take 2-4 hours — don't try to chip it off, you'll damage the coil). Replace the air filter. Once melted, turn the AC back on. If it freezes again, you need a professional — the cause is likely low refrigerant or a blower issue.
Compressor or Capacitor Problems
The compressor is the heart of your AC — it circulates refrigerant and pressurizes it for cooling. Capacitors start the compressor and fan motors.
Capacitor failure (common, $120–$300 repair): If the outdoor fan runs but the compressor doesn't start, or if you hear a humming sound followed by a click, the start capacitor may have failed. This is one of the most common AC repairs and is relatively inexpensive.
Compressor failure (expensive, $1,500–$3,500): Signs include the AC not cooling at all, loud noises from the outdoor unit, or the breaker tripping when the AC starts. If your system is over 10 years old, compressor failure often means it's time to replace the entire system rather than repair.
Signs of compressor problems:
- AC runs but doesn't cool at all
- Loud banging or rattling from the outdoor unit
- Breaker trips when AC starts
- Outdoor unit fan runs but compressor is silent
Ductwork Issues
Sometimes the AC is working fine but the cold air isn't reaching your rooms. Ductwork problems can cause this:
- Leaky ducts: Disconnected or cracked ducts let cold air escape into the attic or walls before it reaches your vents. This can waste 20-30% of your cooling. Look for disconnected joints in accessible ducts and seal with metal tape (not duct tape).
- Poor insulation: Uninsulated ducts in hot attics lose cooling capacity. Adding duct insulation is a cost-effective improvement.
- Blocked ducts: Furniture over floor vents, closed dampers, or collapsed flex ducts can block airflow to specific rooms.
- Undersized ducts: If your AC was upgraded without upgrading the ducts, the ducts may not handle the increased airflow.
Professional duct inspection costs $200–$500 and can identify issues that are reducing your system's efficiency.
When to Call a Professional
Call an HVAC professional if:
- You've checked the filter, thermostat, and breaker, and the AC still doesn't cool
- You see ice on the coils or refrigerant lines
- You hear unusual sounds (grinding, squealing, banging) from the indoor or outdoor unit
- The AC short cycles (turns on and off every few minutes)
- Your energy bills have spiked without a change in usage
- The outdoor unit won't start at all
- You smell burning or electrical odors
When you call, describe the symptoms specifically — this helps the technician diagnose the problem faster, potentially reducing labor costs. And always verify the technician is EPA-certified for refrigerant work.
Key Takeaways
When your AC isn't cooling, start with the simple DIY checks — filter, thermostat, breaker, and vents. These solve a surprising number of problems at no cost. If those don't work, the issue is likely refrigerant, electrical, or mechanical — all of which require a professional. The key is not to let the problem linger: running an AC that isn't cooling properly can cause further damage, turning a $200 repair into a $2,000 one.
