Short answer: When the AC runs but the house stays warm, Carrier lists the most common causes as a thermostat set wrong, a dirty air filter and a blocked or dirty outdoor unit, all things you can check yourself. A frozen indoor coil, a refrigerant leak or a damaged unit need a technician. If you see ice on the copper lines, turn the system off: running it frozen can damage the compressor.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Air from vents is room temperature | Thermostat on fan “on” or heat, not cool | Set to Cool and fan to Auto | No |
| Weak airflow, house slowly warms | Dirty air filter | Replace the filter | No |
| Outdoor unit covered in grass, leaves or dirt | Blocked condenser coil | Clear debris; gently rinse fins | If it does not help |
| Ice on the copper lines or indoor unit | Frozen evaporator coil | Turn the system off to thaw | Yes |
| Hissing, oily spots, cooling gets worse each week | Refrigerant leak | Turn it off | Yes |
| Outdoor fan not spinning, unit hums | Failed capacitor or fan motor | Turn it off at the breaker | Yes |
Check these first
- Thermostat: set to Cool, fan to Auto, and the temperature a few degrees below the room. Replace thermostat batteries if the display is dim.
- Air filter: turn the system off and look at the filter. If it is gray and clogged, replace it. Make sure supply vents and return grilles are open.
- Outdoor unit: clear grass, leaves and anything stacked against it, then gently rinse the fins with a garden hose (no pressure washer).
- Reset: Carrier suggests setting the thermostat to off, switching off the AC breaker, waiting at least 5 minutes, then turning it back on and setting it to Cool.
Frozen coil
Frost on the copper tubing, weak cooling, or more water than usual at the drain are signs of a frozen evaporator coil. Turn the system off (you can leave the fan on to speed thawing) and replace the filter. If it freezes again, the cause is usually low refrigerant or an airflow problem that a technician must find.
Refrigerant problems
AC systems do not use up refrigerant. If it is low, there is a leak. Carrier warns not to try to fix a refrigerant leak yourself; a licensed technician has to find the leak, repair it and recharge the system.
When to call a professional
- The system still does not cool after the thermostat, filter, outdoor unit and reset checks.
- You see ice that comes back after thawing.
- The outdoor fan does not spin, or the unit hums, clicks or trips the breaker.
- You suspect a refrigerant leak.
What it usually costs
HomeGuide (2026) puts most AC repairs at about $150 to $650. Typical prices: service call $75 to $200, capacitor $100 to $250, condenser fan motor $200 to $700, refrigerant recharge $100 to $600, refrigerant leak repair $200 to $1,500, and compressor replacement $600 to $2,500 or more.
Frequently asked questions
Why is my AC running but not cooling?
The most common reasons are a thermostat setting, a dirty filter or a dirty outdoor unit. If those are fine, a frozen coil, low refrigerant or a failed part is likely.
How long should it take an AC to cool a house?
It depends on the house and the weather. If the temperature does not drop at all after an hour, or keeps rising, something is wrong.
Can a dirty filter stop the AC from cooling?
Yes. A clogged filter restricts airflow and can even freeze the coil.
Related problems
Sources
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.