Why Is My AC Not Cooling?
QUICK ANSWER
Check the thermostat is set to 'cool' and below room temperature. Replace a dirty air filter (the most common cause). Verify the breaker hasn't tripped at the outdoor unit. If the outdoor unit isn't running or refrigerant is low, call an HVAC technician.
An AC that runs but doesn't cool is one of the most common HVAC complaints. The good news is most causes are simple DIY fixes; the bad news is the remaining causes (refrigerant leak, compressor failure) require professional service. The diagnostic approach is to work through the simple causes first before assuming the worst. Here is the troubleshooting order plus when to call a pro.
What are the quick DIY checks?
Run through these in order. First: thermostat. Verify it's set to 'cool' and set lower than the current room temperature. Replace thermostat batteries if it's not powering on. Second: air filter. A dirty filter restricts airflow and can cause the AC to freeze up or stop cooling. Replace immediately if it looks gray or dusty. Third: breaker. Check the breaker for both the indoor air handler and the outdoor condenser unit (often two separate breakers). If tripped, reset; if it trips again, there's a deeper issue. Fourth: vents. Make sure supply vents are open and not blocked by furniture.
Is the outdoor unit running?
Walk outside and check the outdoor condenser unit (the large box with a fan). Listen for the fan running and feel for warm air being blown out the top. If the fan isn't running but the thermostat is calling for cooling: the unit has lost power (breaker tripped, capacitor failed) or the contactor is bad. If the fan runs but no air is blown out: the condenser fan motor may be running but compressor failed. For DIY: confirm power. Beyond that, professional diagnosis is needed. Don't open the condenser unit yourself; it contains high voltage and refrigerant under pressure.
Is the outdoor unit frozen?
If you see ice on the indoor evaporator coil or on the refrigerant lines: the AC has frozen up. Turn off the system and let it thaw completely (4 to 24 hours). Causes: dirty air filter restricting airflow (most common, fix yourself), blocked return vents, low refrigerant (needs pro), faulty blower motor (needs pro), thermostat set too low for too long. Replace the filter, check vents, and let the system fully thaw before running again. If it freezes up again within a few hours, the cause is more serious and needs professional diagnosis.
When should you call a pro?
Call an HVAC technician when: DIY checks haven't fixed the issue, the outdoor unit isn't running despite power being on, the system makes unusual sounds (grinding, screeching), you smell anything unusual (burning, sweet/chemical smell of refrigerant), the indoor coil keeps freezing despite filter replacement, the breaker repeatedly trips, the system is over 10 years old and major repairs would be expensive. Service call cost: 100 to 200 dollars for diagnostics. Common pro repairs: refrigerant leak repair (500 to 1500), capacitor replacement (150 to 400), compressor replacement (1500 to 3000). For very old systems, replacement may be more economical than major repair.
AC cooling failures usually trace to simple causes: thermostat setting, dirty filter, tripped breaker. Run through DIY checks first. The outdoor unit running matters; if it's silent, professional help is usually needed. Frozen coils need thawing and filter replacement. For systems over 10 years old with major repair needs, replacement is often more economical than repair. Annual professional maintenance prevents most cooling failures.
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