Why AC Not Cooling: Common Causes Explained

Why Your AC Is Running But Not Cooling

There are few things more frustrating than an air conditioner that runs but doesn't cool. The good news is that many causes are simple to diagnose and fix yourself. This guide walks you through the most common reasons your AC isn't cooling and how to address each one.

Quick Diagnostic Checklist

Before calling a technician, check these common issues:

1. Is the thermostat set correctly?
2. Is the filter dirty?
3. Are all vents open and unblocked?
4. Is the outdoor unit running?
5. Is there ice on the refrigerant lines?

Common Causes and Solutions

1. Dirty Air Filter (Most Common)

The Problem:

• Clogged filter restricts airflow through the system
• Less air reaches the evaporator coil
• Coil can't absorb heat efficiently
• AC runs but room stays warm

Signs:

• Weak airflow from vents
• AC runs longer than usual
• Filter looks gray or matted when inspected

Solution:

1. Turn off AC
2. Remove and inspect filter
3. Clean or replace filter
4. Run AC for 30 minutes and check cooling

2. Thermostat Issues

Common Problems:

• Set to "fan only" instead of "cool"
• Temperature set higher than room temperature
• Dead batteries (for battery-powered thermostats)
• Thermostat located in hot spot, causing incorrect reading
• Wiring issue or miscalibration

Solution:

1. Verify mode is set to "cool"
2. Set temperature at least 3°C below current room temp
3. Replace batteries if applicable
4. Try lowering set point significantly to test response

3. Low Refrigerant (Refrigerant Leak)

The Problem:

• Refrigerant is the substance that absorbs heat
• Low levels reduce cooling capacity
• Refrigerant doesn't get "used up"—low levels indicate a leak

Signs:

• Ice forming on refrigerant lines or outdoor unit
• Hissing or bubbling sound near AC
• AC runs constantly but doesn't cool well
• Higher electricity bills

Solution:

• Requires professional diagnosis and repair
• Technician must find and fix leak before recharging
• DIY is not possible—refrigerant handling requires certification

4. Frozen Evaporator Coils

The Problem:

• Ice builds up on evaporator coils
• Ice blocks airflow and heat transfer
• AC blows room-temperature or slightly cool air

Causes:

• Dirty filter restricting airflow
• Low refrigerant
• Blocked vents
• Blower motor failure
• Running AC when outdoor temp is too low

Solution:

1. Turn off cooling, switch to fan-only mode
2. Let ice thaw completely (2-4 hours)
3. Check and clean filter
4. Run AC normally—if ice returns, call technician

5. Dirty Condenser Coils (Outdoor Unit)

The Problem:

• Outdoor unit releases heat from refrigerant
• Dirty coils can't release heat efficiently
• System struggles to cool

Signs:

• Outdoor unit looks dirty, covered with debris
• AC works but cooling is reduced
• Outdoor unit may be hotter than normal

Solution:

1. Turn off AC
2. Clear debris from around unit (leaves, grass clippings)
3. Gently spray coils with garden hose (from inside out)
4. Ensure 2-foot clearance on all sides

6. Compressor Not Running

The Problem:

• Compressor is the heart of the cooling system
• If it doesn't run, no cooling occurs
• Fan may still blow room-temperature air

Signs:

• Indoor fan blows but air isn't cold
• Outdoor unit fan may run but compressor is silent
• Clicking sound but compressor doesn't start

Possible Causes:

• Capacitor failure (common)
• Compressor motor burnout
• Electrical issue
• Overload protection triggered

Solution:

• Check circuit breaker for outdoor unit
• If breaker trips repeatedly, don't reset—call technician
• Capacitor and compressor issues require professional repair

7. Blocked or Closed Vents

The Problem:

• Furniture or objects blocking supply vents
• Closed vents in some rooms
• Air can't circulate properly

Solution:

1. Walk through home and check all vents
2. Open all closed vents (closing vents doesn't save energy)
3. Move furniture at least 6 inches from vents
4. Check return air vents are unobstructed

8. Wrong Size AC

The Problem:

• Undersized AC can't cool the space adequately
• Runs constantly but never reaches set temperature
• Problem worsens in extreme heat

Signs:

• AC runs non-stop during hot days
• Room never reaches set temperature
• Issue exists since installation or room modification

Solution:

• Use AC sizing calculator to verify correct size
• If undersized, consider supplemental cooling or replacement
• Improve insulation to reduce cooling load

When to Call a Professional

Call Immediately If:

• You smell burning or see smoke
• Ice keeps returning after thawing
• You hear hissing (refrigerant leak)
• Circuit breaker keeps tripping
• Outdoor unit doesn't run at all

Schedule Service If:

• DIY fixes don't resolve the issue
• Cooling is significantly reduced
• Bills spike without explanation
• System is more than 10 years old

Troubleshooting Flow Chart

1. AC won't turn on: Check power → Check thermostat → Check breaker
2. AC runs, no cold air: Check filter → Check thermostat mode → Check outdoor unit
3. Weak cooling: Clean filter → Check vents → Clean outdoor unit
4. Ice on lines: Turn off, thaw, check filter → If returns, call pro
5. AC runs constantly: Undersized? → Extreme heat? → Call pro if persists

Conclusion

Most "AC not cooling" problems start with simple issues: dirty filter, thermostat settings, or blocked vents. Check these first before calling for service. For refrigerant leaks, frozen coils that keep returning, or compressor problems, professional diagnosis is necessary. Regular maintenance prevents most cooling issues before they start.