Troubleshooting
AC Not Cooling? Step by Step Fixes
Quick answer
When an AC stops cooling, work in order: thermostat settings, breakers, air filter, indoor coil ice check, then the outdoor unit. If the outdoor fan or compressor is not running, or the system runs constantly without dropping temperature, the fault is refrigerant or electrical and needs a technician.
The 10 minute homeowner diagnostic
Follow this sequence exactly. It mirrors what a technician checks first and resolves about one in three cases without a service call.
Thermostat
Set to Cool, fan Auto, setpoint at least 3 degrees below room temperature. Replace batteries if the display is blank or dim.
Power
Check both breakers and the outdoor disconnect box near the condenser. Also check the float switch on the indoor unit; a full condensate pan shuts cooling down on purpose.
Filter
Hold the filter up to light. If you cannot see light through it, replace it now.
Indoor coil
Look for frost on the copper lines or coil. If frozen, switch to Off with fan On for 2 to 4 hours to thaw before further testing.
Outdoor unit
Is the fan spinning? Is the big line cold and sweating? A silent condenser, a humming unit with a stopped fan, or a warm suction line all point to specific failures a tech can fix quickly.
Matching symptoms to likely repairs
What you observed in the diagnostic points directly at the failed component and its typical cost.
| Symptom | Likely cause | Typical repair cost |
|---|---|---|
| Outdoor unit silent, indoor blower runs | Capacitor, contactor, or breaker | $150 to $400 |
| Outdoor unit hums, fan not spinning | Fan motor or capacitor | $200 to $700 |
| Runs constantly, weak cooling | Low refrigerant or dirty coils | $225 to $1,600 |
| Ice on indoor coil or lines | Airflow restriction or low charge | $100 to $1,600 |
| Short cycles every few minutes | Oversizing, thermostat, or pressure trips | $150 to $600 |
| Loud clunk then silence | Compressor failure | $1,300 to $2,900 |
When not cooling means replace instead
If your system is over 12 years old, uses discontinued R-22 refrigerant, or this is its second major failure in two seasons, put the repair quote through the 5,000 rule: multiply system age by repair cost, and above 5,000, replacement is usually the smarter spend. New systems also unlock 20 to 40 percent energy savings and current tax credits.
Frequently asked questions
Why is my AC freezing up?
Ice forms when the evaporator coil gets too cold, caused either by restricted airflow, from a clogged filter, blocked vents, or a failing blower, or by low refrigerant pressure from a leak. Thaw it fully, replace the filter, and if it refreezes call a technician for a charge check.
How long should an AC take to cool a house?
A correctly sized system lowers the house about 1 degree every 10 to 20 minutes in moderate weather. On a 95 degree day, expect it to hold roughly 20 degrees below outdoor temperature rather than reach an arbitrary setpoint quickly.
Should I turn off a broken AC?
Yes, if it is frozen, making grinding or electrical noises, or tripping breakers repeatedly. Continuing to run a faulted system can convert a $300 repair into a compressor replacement. Fan only mode is safe to keep air moving until service.
Related guides
- Why Is My AC Blowing Warm Air?The 7 most common reasons an air conditioner blows warm air, which ones you can fix yourself, and when to call a professional.
- Frozen Evaporator Coil: Causes and FixesWhy AC coils freeze into a block of ice, how to thaw one safely, and how to keep it from happening again.
- How Long Does AC Repair Take?Realistic timelines for every common AC repair, from 30 minute capacitor swaps to multi day compressor jobs, plus what delays repairs and how to speed yours up.
- Heat Pump Not Heating? Causes and FixesWhy heat pumps blow lukewarm air, ice over, or fail in cold snaps, what is normal defrost behavior versus real failure, and repair costs.