HVAC Not Cooling? 8 Common Causes and Fixes

Scope of this guide: Use this guide for a system that runs but does not lower indoor temperature. The warm-air guide focuses specifically on air coming from the vents at the wrong temperature.

Your air conditioner is running but the house isn't getting cold. It's one of the most frustrating situations in summer - the system seems to be working, but the temperature won't drop. Here are the eight most common reasons an AC runs without cooling, from the simple to the serious.

1. Clogged Air Filter

The most common cause of poor AC performance. A dirty filter restricts airflow across the evaporator coil, reducing the system's ability to absorb heat from your home's air.

Check it: Pull out your filter (usually in the return air grille or air handler). If it's gray or dark brown, it needs replacing.

Fix: Replace the filter ($10-$20). This alone often restores normal cooling. Change filters every 1-3 months going forward.

2. Thermostat Set to "Fan" Instead of "Cool"

The fan runs whether the AC is cooling or not. If the thermostat is set to "Fan On" instead of "Auto," air circulates but no cooling happens.

Check it: Look at the thermostat settings. It should be set to "Cool" mode with the fan on "Auto."

Fix: Switch to Cool mode, set the target temperature below the current room temperature.

3. Refrigerant Is Low (Leak)

Refrigerant is the chemical that absorbs heat from your home's air. If there's a leak, the system can't transfer heat effectively. Low refrigerant feels like warm or barely cool air coming from the vents.

Check it: This requires gauges - a DIY check isn't possible. Signs include ice forming on the refrigerant line or evaporator coil.

Fix: A licensed HVAC technician must locate the leak, repair it, and recharge the refrigerant. Simply adding refrigerant without fixing the leak is a temporary solution that wastes money.

Cost: $200-$1,500 depending on leak location and refrigerant type.

4. Dirty Condenser Coils (Outdoor Unit)

The condenser unit (outside) rejects heat from your home into the outdoor air. If the coils are coated in dirt, grass clippings, or debris, heat can't escape efficiently.

Check it: Look at the outdoor unit. The fins on the sides should be visible and clean. If they're packed with debris or vegetation, airflow is restricted.

Fix: Turn off the power and gently rinse the coils with a garden hose from the inside out. Clear all vegetation within 18 inches of the unit. An annual professional coil cleaning covers this.

5. Frozen Evaporator Coil

If airflow is too restricted (clogged filter, blocked vents) or refrigerant is low, the evaporator coil can freeze. Ice on the coil stops it from absorbing heat.

Check it: Turn off the AC and set the fan to "On" to thaw the coil for 1-2 hours. Then check the filter and vents.

Fix: Address the underlying cause - usually a clogged filter or low refrigerant. If it keeps freezing, call a technician.

6. Blocked or Closed Vents

If supply or return vents are closed or blocked by furniture, airflow to the system is reduced. This doesn't always stop the AC, but it reduces how well the system can cool the space.

Check it: Walk through the home and verify all supply and return vents are open and not obstructed by furniture, rugs, or boxes.

Fix: Open all vents, move furniture if needed. Closing vents in unused rooms does not save energy and can cause pressure problems in the duct system.

7. Failing Capacitor

The capacitor starts and runs the compressor and fan motors. When it fails, the compressor may not start (you'll hear a hum or clicking but no cooling), or the fan may run without the compressor.

Check it: This requires a technician. If you hear the outdoor unit humming but not starting, or if the fan runs but the unit doesn't cool, this is a common culprit.

Fix: Capacitor replacement is a fast, inexpensive repair - typically $150-$300. A common issue, especially in hot climates.

8. Compressor Failure

The compressor is the heart of the cooling system. When it fails, the system runs but doesn't cool at all. Compressor failure is the most serious AC problem and often makes replacement more cost-effective than repair.

Check it: A technician can diagnose this. Signs: AC runs normally but produces zero cooling; outdoor unit is warm to the touch rather than blowing hot air.

Fix: Compressor replacement costs $1,200-$2,500. On a system over 10 years old, full replacement often makes more financial sense.

When to Call a Technician

Issues 1, 2, 4, 5, and 6 can be diagnosed and sometimes fixed by a homeowner. Issues 3, 7, and 8 require a licensed HVAC technician with proper tools and refrigerant certification.

If your AC is running but not cooling and a filter change doesn't fix it, call a tech rather than letting the system run - operating with a serious problem can cause additional damage.

Bottom Line

An AC that runs but doesn't cool is usually one of eight problems: dirty filter, wrong thermostat setting, refrigerant leak, dirty condenser, frozen coil, blocked vents, failing capacitor, or compressor failure. Start with the simple checks before calling for service.

Sources and editorial notes

Prices and permit requirements vary by location, property, equipment, and scope. Treat price ranges as planning estimates and confirm current requirements with the relevant authority and contractor.

• California Contractors State License Board - when a contractor license is required
• US Department of Energy - Energy Saver

Related guides

• Air Conditioner Blowing Warm Air: What It Usually Means
• How to Reduce Indoor Dust Without Buying the Wrong HVAC Filter
• Garbage Disposal Not Working: Common Fixes Before Calling a Plumber

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