Short answer
Set the thermostat to "Cool" a few degrees below room temperature and replace a dirty air filter. Check the AC breaker in the panel and the disconnect box by the outdoor unit, and reset a tripped one only once. With the power off, clear leaves around the outdoor unit and rinse its fins gently. If you see ice on the copper lines, set the system to "Off" with the fan "On" and call a technician.
Safety first
In extreme heat, no AC can be dangerous for babies, older adults and people with health conditions: move them to a cooler place while you wait. Turn off the power at the outdoor disconnect before clearing anything around the unit, and never open the electrical panels of the unit.
How to check an AC that is not cooling
1Warm air from the vents
The fan runs but the air is not cold. These checks fix many cases and take about 20 minutes.
Read all the steps
- Warm air from the vents. The fan runs but the air is not cold. These checks fix many cases and take about 20 minutes.
- Thermostat on Cool, a few degrees lower. Set it to "Cool", a few degrees below room temperature, with the fan on "Auto". Blank or dim screen? Replace the batteries.
- Replace a dirty air filter. Slide out the filter at the return grille or the indoor unit. If it is gray and clogged, put in a new one of the same size.
- Check the AC breakers. Look at the AC breaker in the panel and the disconnect box by the outdoor unit. Reset a tripped one once. Never open the unit's panels.
- Power off, then clean around the outdoor unit. Switch the disconnect box to "Off". Clear leaves and grass two feet around the unit and rinse the fins gently with a garden hose. No pressure washer.
- Power on and give it 15 minutes. Turn the disconnect back on. Still warm, ice on the copper lines, or a hum with the fan stopped? Set the thermostat to "Off" and call.
- Still not fixed? Call an HVAC contractor Especially if: the outdoor fan does not spin, or the unit hums, clicks or smells burnt.
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Ice on the lines or the indoor coil
Ice means the system is not moving enough air or is low on refrigerant. Set the thermostat to "Off" and the fan to "On" so the ice melts (it can take a few hours; put towels under the indoor unit). Replace the filter. If it freezes again, call a technician: low refrigerant means there is a leak, and adding refrigerant without fixing the leak only buys a few days.
Water around the indoor unit
The AC removes moisture from the air and drains it through a small pipe. If that drain clogs, water backs up and many systems shut off by themselves to avoid a flood. A technician can clear the drain line and the pan; it is usually a quick fix.
What not to do
- Do not add refrigerant yourself or use a "recharge kit". Handling refrigerant requires certification, and a low charge means a leak.
- Do not use a pressure washer or a stiff brush on the fins: they bend easily. A garden hose on a gentle setting is enough.
- Do not chip ice off the lines or the coil.
- Do not open the electrical panels of the outdoor unit. Parts inside can hold a charge even with the power off.
- Do not block vents or the return grille with furniture or rugs: the system needs the airflow.
When to call an HVAC technician
- The outdoor fan does not spin, or the unit hums, clicks or smells burnt.
- There is ice on the copper lines, or it comes back after you thaw it and replace the filter.
- The breaker trips again after one reset.
- It runs all day and the house still does not cool.
- Someone at home is at risk from the heat.
Frequently asked questions
Why is my AC running but not cooling?
Most often it is a clogged air filter, a dirty outdoor unit, a thermostat setting or a tripped breaker, and you can check those yourself. Low refrigerant from a leak, a failed capacitor or a bad fan motor need a technician.
Can I add refrigerant myself?
No. Handling refrigerant requires certification, and low refrigerant means there is a leak that needs to be found and fixed.
How long should I wait for a frozen AC to thaw?
Usually one to a few hours with the thermostat on "Off" and the fan on "On". Do not chip the ice. If it freezes again with a new filter, call a technician.
How often should I change my AC filter?
Check it every month while you use the AC and replace it when it looks gray, usually every 1 to 3 months. Homes with pets or a lot of dust need it more often.
Can I hose off my outdoor AC unit?
Yes, gently. Turn off the power at the disconnect box, then rinse the fins from the top down with a garden hose. Do not use a pressure washer: it bends the fins.
Is no AC an emergency?
In extreme heat it can be, especially for babies, older adults and people with health conditions. Use the "24/7 emergencies" filter to find technicians who answer now.