AC Not Cooling? What to Check Before You Call
Short answerAn AC that runs but does not cool usually has a dirty filter, a wrong thermostat setting, a dirty outdoor coil or low refrigerant from a leak. Set the thermostat to cool, change the filter and clear around the outdoor unit. If it still does not cool, shut it off and call an HVAC technician.
Safety first
- Turn off the system at the thermostat before changing a filter. Turn off the breaker before cleaning around the outdoor unit.
- Do not open the outdoor unit or electrical panels. Refrigerant work is for a trained technician.
- If you see ice on the coil or lines, turn the system off so the compressor is not damaged.
Likely causes
| Cause | How to tell | What fixes it |
|---|---|---|
| Dirty air filter | Weak airflow from the vents and a gray, clogged filter. | Replace the filter, and check it monthly. |
| Thermostat setting or batteries | The system is set to heat, the fan is set to on, or the screen is blank. | Set it to cool, set the fan to auto, set the temperature below room temperature, and replace the batteries. |
| Dirty outdoor coil | Debris, plants or a dirty coil at the outdoor unit, or it runs hot and shuts down. | Clear plants to at least 2 feet from the sides. A pro cleans the coil. |
| Clogged drain line | Water near the indoor unit, or the indoor unit shuts off. | A pro clears the line. |
| Low refrigerant or leak | Air is cool but not cold, or ice forms on the lines. | A technician finds and repairs the leak, then recharges the system. |
| Leaking ducts or a failed control | Some rooms get little air, or the unit will not turn on at all. | A pro seals the ducts or checks the electrical connections. |
Why this is a job for a professional
- The thermostat and filter checks did not help.
- You see ice, hear buzzing or clicking, or the breaker trips.
- Ask: what is the refrigerant level, and where is the leak? Did you check airflow and coils? Is it under warranty? Is repair or replacement a better value, and can I see both prices in writing?
What it costs to have fixed
AC Replacement is estimated at $3,800 to $9,050 nationally. See your state
Common questions
Why is my AC not blowing cold air?
Start with the thermostat and filter, since those are the easiest to fix. After that, the most common causes are a dirty coil, a clogged drain line and low refrigerant, which a technician has to diagnose.
How often should I change my AC filter?
ENERGY STAR says to check it monthly. Trane suggests changing it every 1 to 3 months, depending on the filter.
Can I add refrigerant to my AC myself?
No. Low refrigerant means a leak or a bad charge at installation, and too much or too little hurts the system. A trained technician should repair the leak and recharge it.
Why is my AC set to 72 but reads 78?
The AC may simply be losing to the heat, or something is limiting it. Bryant says extreme outdoor heat, poor insulation or an older, undersized unit can make the target hard to reach, and it lists low refrigerant, dirty coils or filters as likely causes. Change the filter, close blinds on hot days, and call a technician if it still falls short. Source
Why is my AC blowing warm air?
Warm air from the vents can mean the outdoor unit is not running. Lennox explains that the indoor fan then moves air through without cooling it. Check the breaker once, and call a professional if it trips again right away or the outdoor unit buzzes or hums. Source
How do I know if my AC refrigerant is low?
Trane lists low refrigerant as a possible reason for warm air from the vents and says refrigerant leaks can happen over time. It says you will need an expert to evaluate the system, repair the cause of the leak and restore the refrigerant level. Source
How can I make my AC colder?
Setting a lower temperature will not cool the house any faster, according to American Standard. It suggests ceiling or portable fans for a windchill effect, regular tune-ups and filter changes, and keeping the setting within 20 degrees of the outdoor temperature. Source
Related repairs
- AC Freezing Up? Why It Happens and What to Do
- Furnace Won't Turn On? Safe Checks Before You Call
- Smell Gas in Your House? Leave Now, Then Call
- Thermostat Not Working? Blank Screen and Other Fixes
Last verified 2026-11-01
Sources
- U.S. Department of Energy: Energy Saver 101, Home Cooling — Common problems: refrigerant leaks, dirty filter, thermostat, drain, ducts and electric control failure, and leaks needing a trained technician
- ENERGY STAR: Maintenance Checklist — Monthly filter checks, what a contractor does at a tune-up, dirty coils, and refrigerant levels
- Trane: AC Not Blowing Cold Air — Order of causes, thermostat checks, filter every 1 to 3 months, 2 foot plant clearance, and what to leave to a pro
General information, not professional advice. Conditions, codes and prices vary. Turn off power, gas or water before any work, and hire a licensed professional when in doubt. Cost figures are our estimates, not quotes.