● Check Before DIY

Last updated: March 2026 · Reviewed by the Toolbox Team

AC Not Cooling? Check These Things First

Before you spend $150+ on an HVAC service call, check the basics. A dirty air filter restricts airflow and is the single most common cause of AC not cooling properly. It takes 30 seconds to check and a $5-15 filter to fix.

AC not coolingAir conditioner not coolingAC running but not coolingAC blowing warm airCentral AC not workingAC won't cool house

Key takeaway: Before spending $150+ on an HVAC service call, replace the air filter — a clogged filter is the single most common cause of AC problems. It takes 30 seconds to check and a $5-15 filter to fix.

What's Happening

Central AC systems work by circulating refrigerant between an indoor evaporator coil (absorbs heat) and an outdoor condenser unit (releases heat). A blower fan pushes air across the indoor coil and through your ducts. When any part of this system is blocked or failing — dirty filter, frozen coil, blocked condenser, low refrigerant, or thermostat issue — cooling drops.

What to Check

Check and replace the air filter. Find your return air vent (usually a large grille on a wall or ceiling) or the filter slot at your air handler. Pull out the filter. If it's gray, clogged, or you can't see light through it, replace it. A clogged filter can freeze the evaporator coil and shut down cooling entirely.
Check the thermostat settings. Make sure it's set to COOL (not HEAT or FAN ONLY), the temperature is set below current room temp, and the fan is set to AUTO. Try setting it 5 degrees below room temperature and wait 5 minutes.
Check the outdoor condenser unit. Go outside and look at the unit. Is it running? You should hear the compressor humming and the fan spinning. If it's not running, check the disconnect switch near the unit and the breaker in your panel. Clear any debris, leaves, or vegetation within 2 feet of the unit.
Check for ice on the refrigerant lines. Look at the copper lines running between the indoor and outdoor units. If they're covered in ice or frost, the system is freezing up — usually from a dirty filter or low refrigerant. Turn the AC off and run the fan only for 2-3 hours to thaw, then replace the filter and try again.
Check your vents. Walk through the house and make sure all supply vents are open and unblocked. Closed vents in unused rooms reduce overall system performance and can cause freezing.

DIY Cost

$5–15

Pro Cost

$150–600

Call HVAC Pro If

Replacing the filter and cleaning the condenser didn't restore cooling after 24 hours.
The system freezes up repeatedly (likely low refrigerant — a leak that needs professional repair).
The outdoor unit isn't running and the breaker keeps tripping.
You hear hissing or bubbling sounds from the refrigerant lines (refrigerant leak).
The system blows warm air continuously.

Frequently Asked Questions

How often should I change my AC filter?

Every 1-3 months depending on usage, pets, and allergies. During heavy summer use, check monthly. A dirty filter is the #1 cause of AC failure.

Why does my AC run but not cool?

Most common causes: dirty filter, frozen evaporator coil, dirty condenser unit outside, low refrigerant, or thermostat set incorrectly. Check the filter first.

What does it mean if my AC lines are frozen?

Ice on the refrigerant lines means the system is freezing up — usually from restricted airflow (dirty filter) or low refrigerant. Turn off the AC, run fan only for 2-3 hours to thaw, then replace the filter.

Why is my AC still not cooling after I changed the filter?

If a new filter didn't fix it, the next likely causes are: a dirty outdoor condenser unit (clean the fins with a hose), a frozen evaporator coil (turn off cooling, run fan to thaw), or low refrigerant. Low refrigerant requires an HVAC technician — it can't be topped off without finding the leak first.

Can low refrigerant cause AC to not cool?

Yes — low refrigerant is one of the most common reasons an AC runs continuously but can't reach the set temperature. You may also notice ice on the lines or a hissing sound. This always requires a licensed HVAC technician to diagnose and repair — refrigerant cannot legally be added without a certified professional.

How much does it cost to fix AC not cooling?

It depends entirely on the cause. A dirty filter: $5–$15 to fix yourself. A dirty condenser cleaning: free (DIY with a hose). A refrigerant recharge: $150–$400 depending on refrigerant type. A failed compressor: $800–$2,500. Always check the free DIY steps first before calling a tech.

Not Sure If It's a Filter or a Refrigerant Issue?

Record a short video of your AC system — indoor and outdoor unit — and Toolbox diagnoses whether it's something you can fix yourself or needs a certified HVAC technician.

Coming soon to theApp Store Coming soon toGoogle Play

First diagnosis free — no credit card.

Need a Pro? Find One on Thumbtack

If the filter is clean, the condenser is clear, and the AC is still not cooling, you likely have a refrigerant, compressor, or coil issue — compare vetted local HVAC pros.

Find a Local HVAC Pro →