Solution · Home, Lifestyle & Leisure · Updated 2026-11-01 · Reviewed by Monirujjaman Monjil
AC not cooling? What to check before you call a technician
Check the simple things first: that the thermostat is set to cool at a lower temperature than the room and has working batteries, that the air filter isn't clogged, that the drain isn't blocked, and that vents and the outdoor unit aren't obstructed. If it still doesn't cool, especially if you suspect low refrigerant, a leak or electrical problems, call a trained technician. The U.S. Department of Energy lists low or leaking refrigerant as a job for a technician, who should repair the leak and recharge the system.
The problem
What it looks like
The air conditioner is running but the room stays warm, the air from the vents isn't cold, or the unit won't turn on at all.
Who it affects
Homeowners and renters with central, split or window air conditioners, especially at the start of the hot season.
What it costs you
An uncomfortable or unsafe home in hot weather, higher energy bills, and damage to the system if a problem is left running.
Why it happens
The U.S. Department of Energy lists common causes: a dirty filter restricting airflow, a thermostat that isn't set properly or isn't reading the right temperature, a blocked drain, a knocked-out sensor on window units, leaking or blocked ducts, worn electrical controls, and low or leaking refrigerant.
Many 'broken' air conditioners have simple causes. The Department of Energy describes cleaning or replacing filters as the quickest way to save energy on home cooling; a clogged filter restricts airflow and reduces the unit's ability to cool.
Some problems are not DIY. Refrigerant work requires a trained technician (in the U.S., technicians who handle refrigerants must be EPA-certified), and electrical faults can be dangerous.
Turn the power off at the switch or breaker before touching anything inside or around the unit.
Common mistakes
- Turning the thermostat very low to cool faster; it doesn't work that way.
- Running the unit for weeks with a filter that is visibly dirty.
- Blocking the outdoor unit with plants, covers or stored items.
- Trying to add refrigerant yourself.
- Ignoring ice on the coils or water leaking indoors.
How to fix it, step by step
- Check the thermostat
Make sure it is set to cool, the set temperature is below the room temperature, and the display is working. Replace the batteries if it has them.
- Check the power
Look for a tripped breaker or switched-off disconnect. If the breaker trips again, stop and call a professional.
- Check the air filter
Turn the system off, remove the filter and look at it. Replace or clean it if it's dirty. The DOE recommends routinely replacing or cleaning filters.
- Check airflow
Make sure supply vents and return grilles inside are open and not blocked by furniture, and that the outdoor unit has clear space around it.
- Check the drain
If there's water around the indoor unit or the system has shut off, the condensate drain may be blocked.
- Window units: check the sensor
On window units, the thermostat sensor can be knocked out of position. The DOE suggests carefully bending the wire that holds it back into place. Unplug the unit first.
- Call a technician for anything else
Ice on the coils, hissing or bubbling sounds, warm air despite a clean filter, or repeated tripping usually mean refrigerant, electrical or component problems for a trained technician.
Doing it without an app
You do not need to download anything to solve this. Any of these work:
Keep heat out while you wait
Close curtains and blinds on sunny sides, avoid using the oven, and use fans to move air.
Annual maintenance
The DOE suggests checking the evaporator coil every year and cleaning it as necessary. A yearly service before summer catches many problems early.
Tools that can help
- Replacement filters in the right size
- A smart or programmable thermostat
- Fin combs for straightening bent coil fins
Questions
How often should I change my AC filter?
Follow the manufacturer's guidance and check it regularly during heavy use. Replace or clean it whenever it looks dirty.
Can I add refrigerant myself?
No. Low refrigerant usually means a leak that needs repairing, and refrigerant work requires a trained, certified technician.
Why is there ice on my AC?
Restricted airflow (often a dirty filter) or low refrigerant can cause ice. Turn the system off to let it thaw, check the filter, and call a technician if it returns.
I'm an HVAC technician. Is there a tool for this?
Our Coolbench app is built for technicians, with a charging assistant for superheat and subcooling, PT charts and HVAC calculators.
Sources
- Energy Saver 101: Everything You Need to Know About Home Cooling, U.S. Department of Energy, 2016
Checked 2026-11-01. Found a mistake? Tell us. Read our editorial policy.
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