Short answer: Start with the simple checks: the thermostat set to Cool and Auto, a clean air filter, the breaker for the outdoor unit, and a clear, clean outdoor coil. If the lines or indoor coil are icing up, every vent blows warm air, or you hear hissing, the likely cause is low refrigerant or a failing part, which needs an HVAC technician.
Quick checks you can do safely
- Thermostat. Set it to Cool, the fan to Auto, and a temperature a few degrees below the room. Replace the batteries if it uses them.
- Air filter. A clogged filter chokes airflow. Energy.gov suggests cleaning or replacing filters every month or two during the cooling season, and more often with pets or dust.
- Breakers and switches. Check the breakers for the outdoor unit and the indoor air handler, and the service switch near the indoor unit.
- Outdoor unit. Clear leaves and trim plants back so air can flow. With the power off at the outdoor disconnect, rinse the coil gently with a garden hose.
- Vents, windows and doors. Make sure supply and return vents are open and unblocked, and that windows and exterior doors are closed.
8 common causes
| Cause | Signs | Who fixes it |
|---|---|---|
| Thermostat setting or failure | Fan runs but no cooling; blank display | You (settings, batteries) or a technician |
| Dirty air filter | Weak airflow; dusty filter | You |
| Dirty outdoor coil | Outdoor unit caked with dirt; runs nonstop | You (gentle rinse) or a technician |
| Frozen indoor coil | Ice on the copper line or coil; water near the air handler | Turn cooling off to thaw; a technician finds the cause |
| Low refrigerant (a leak) | Warm air, icing, hissing, long run times | A technician must find and repair the leak |
| Clogged condensate drain | Water near the indoor unit; system shuts off | You (clear the line) or a technician |
| Failed capacitor or contactor | Outdoor fan or compressor will not start; humming | A technician |
| Leaky or undersized ducts | Some rooms cool, others do not | A technician or duct specialist |
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AC or furnace not working right?
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Why adding refrigerant is not a fix
If your system is low on refrigerant, Energy.gov notes that it was either undercharged at installation or it leaks, and if it leaks, simply adding refrigerant is not a solution. A technician should find and repair the leak, then charge the system to the manufacturer’s specification.
When to call an HVAC technician
- You see ice on the refrigerant lines or the indoor coil.
- The outdoor unit hums but the fan does not spin, or it will not start at all.
- The breaker trips again after you reset it.
- You smell burning, or hear grinding, squealing or hissing.
- The system is old and repairs keep adding up.
Frequently asked questions
Should I keep running an AC that is not cooling?
If it is icing up or making unusual noises, switch cooling off at the thermostat to protect the compressor and leave the fan on to help melt the ice. Then call a technician.
How often should an AC be serviced?
Energy.gov recommends regular maintenance of the filters, coils and fins, and notes that annual maintenance can improve comfort and efficiency. Most homeowners book a professional tune-up before each cooling season.