AC Running But Not Cooling? Here’s What Might Be Wrong

Your thermostat says 72. Your AC is running. Air is coming out of the vents. And somehow your house still feels miserable. 

If your AC is running but not cooling, the issue could be something simple like a clogged air filter, or something more serious like a refrigerant leak or failing compressor.

Here are the most common reasons we see in Los Angeles and the San Fernando Valley, along with what you can safely check before calling for service.

1. Dirty Air Filter

A clogged air filter is one of the most common reasons an AC system struggles to cool properly.

Restricted airflow makes it harder for your system to move enough conditioned air through the home. In some cases, it can even cause the evaporator coil to freeze.

• Weak airflow from vents
• Rooms cooling unevenly
• AC running longer than normal
• Ice forming near the unit

What to do: 

Check the filter. If it looks dirty, replace it. 

2. Low Refrigerant or Refrigerant Leak

Your AC doesn’t consume refrigerant.

If refrigerant is low, there’s almost always a leak somewhere in the system.

Without enough refrigerant, the system can’t absorb and remove heat effectively.

• Common signs: 
• AC blowing warm or barely cool air
• Ice on refrigerant lines
• Hissing sounds
• Rising utility bills
• Long run times

What to do: 

This needs professional diagnosis. Simply adding refrigerant without fixing the leak is a band-aid.

3. Frozen Evaporator Coil

If airflow is restricted or refrigerant is low, the evaporator coil can freeze over.

Once that happens, cooling performance tanks. 

Signs: 

• Ice on copper refrigerant lines
• Water near indoor equipment
• Weak airflow
• Warm air

What to do: 

Turn cooling off. Switch the fan to ON and let the system thaw before scheduling service. 

4. Thermostat Problems

Sometimes the HVAC system is fine, and the thermostat is the issue.

Check for: 

• Incorrect mode (HEAT instead of COOL)
• Dead or low batteries
  
• Programmed schedule overrides
• Sensor issues
• Calibration problems

This is especially common after power outages or battery replacement

5. Outdoor Unit Problems

If your outdoor condenser isn’t doing its job, the system won’t cool.

Possible causes: 

• Failed capacitor
  
• Bad contactor
  
• Condenser fan failure
• Dirty condenser coil
• Electrical issues
• Compressor problems

If the indoor blower runs but the outdoor unit doesn’t, this is a likely culprit. 

6. Ductwork Problems

If some rooms cool while others stay hot, duct issues may be the problem.

Things we see: 

• Disconnected ducts
  
• Crushed duct runs
  
• Major air leakage
• Undersized ductwork
• Poor airflow balancing

This is especially common in older Los Angeles and San Fernando Valley homes where systems have been modified over time.  

7. System is Undersized

Sometimes, nothing is broken.

If your AC runs constantly during hot weather but never catches up, it may simply be undersized for the home. 

This happens more than people realize, especially when systems were replaced based on “same size as the old one.” 

 

What You Can Check Before Calling

Before scheduling service: 

✅ Replace the filter
✅ Confirm thermostat settings
✅ Check the breaker
✅ Make sure outdoor unit is running
✅ Look for ice buildup

If everything checks out and your AC still isn’t cooling, it’s time for a professional diagnosis. 

AC Not Cooling in Los Angeles? We Can Help

At US1 Heating & Air, we diagnose AC cooling issues throughout Los Angeles and the San Fernando Valley.

Our diagnostic includes: 

• System performance testing
• Airflow checks
• Thermostat verification
• Electrical inspection
• Refrigerant diagnostics

And the diagnostic fee is applied towards your repair if you move forward.  

 

Frequently Asked Questions

Why is my AC running but not cooling?

If your AC is running but not cooling, common causes include a dirty air filter, low refrigerant, a frozen evaporator coil, thermostat problems, or outdoor unit failure.

Why is my AC blowing warm air?

Warm air can be caused by refrigerant leaks, frozen coils, electrical issues, thermostat settings, or a failing compressor.

Can a dirty air filter stop my AC from cooling?

Yes. Restricted airflow can reduce cooling performance and may even cause the evaporator coil to freeze.

Should I turn my AC off if it’s frozen?

Yes. Turn cooling off and switch the fan to ON to help thaw the system before scheduling service.