HVAC

AC not cooling? Why your air conditioner runs but won't cool

Updated 2026-06-03 · ~3 min read

Quick answer

When an AC runs but doesn't cool, the cause is almost always airflow or refrigerant. Start with the easy wins: confirm the thermostat is set to COOL and below room temperature, replace a dirty filter, and make sure the outdoor unit is running and clear of debris. If the indoor coil has frozen into a block of ice, turn the system off and let it thaw. Persistent weak cooling — especially with ice or hissing — usually means low refrigerant or a failing component, which is a pro fix.

Common causes

A clogged air filter choking airflow — the most common and most overlooked cause.
A frozen evaporator coil (from low airflow or low refrigerant), which blocks cooling entirely.
A dirty outdoor condenser coil that can't shed heat, often from grass, leaves, or cottonwood.
Low refrigerant from a leak — an AC doesn't 'use up' refrigerant, so low charge means a leak.
A failed capacitor or contactor, or a tripped breaker, so the compressor or fan isn't running.
Thermostat issues — wrong mode, dead batteries, or a bad reading.
A clogged condensate drain that triggers a safety float switch and shuts the system down.

What to check first

Set the thermostat to COOL and a few degrees below the current room temperature; replace its batteries if it's blank.
Check the air filter — a gray, clogged filter alone can stop cooling. Replace it.
Go outside: is the condenser fan spinning? Clear any leaves, grass, or debris from around it.
Look at the indoor unit and refrigerant lines — ice on the coil or the copper line means a frozen coil. Switch to FAN only and let it thaw fully (several hours).
Check the breaker for the AC, and the condensate drain/pan for standing water.

When it's urgent

A no-cool AC isn't usually an emergency for the equipment, but extreme indoor heat is a real health risk for infants, older adults, and pets — relocate or use fans and hydration while you wait for service. If the coil is frozen and water is dripping where it shouldn't, turn the system off to prevent water damage. A burning smell or repeated breaker trips means shut it off and call a pro.

DIY vs. call a pro

You can likely DIY

Resetting the thermostat, replacing the filter, and clearing the outdoor unit.
Thawing a frozen coil by running the fan with cooling off.
Flushing a clogged condensate drain line.

Call a pro for

Refrigerant leaks and recharging — refrigerant is EPA-regulated and requires a licensed tech.
A failed compressor, capacitor, contactor, or fan motor.
A coil that keeps freezing after a fresh filter, or any electrical fault.

Estimated cost range

A diagnostic service call runs $75–$200; a capacitor is $150–$400 installed; a refrigerant leak repair + recharge is $300–$1,500+; a failed compressor can mean $1,500+ (often the moment to weigh replacement).

Varies by market and part. A dirty filter or clogged drain is essentially free to fix yourself — always rule those out first.

How HouseCue helps

HouseCue is a private, homeowner-first app that turns this from a one-time worry into a tracked plan. Snap a photo for an AI diagnosis, upload your inspection report to auto-build a handbook, and get seasonal reminders for your roof, HVAC, water heater, plumbing, and electrical — so nothing slips. Connecting with a pro is always optional and only when you choose.

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Frequently asked questions

Why is my AC running but not cooling the house?

The system is moving air but not removing heat. The usual culprits are a dirty filter or frozen coil (airflow problems) or low refrigerant from a leak. Check and replace the filter first, then look for ice on the indoor coil or refrigerant line.

Why is my AC freezing up?

A coil freezes when airflow is too low (dirty filter, blocked vents, weak blower) or refrigerant is low. Turn the system to FAN only to thaw it completely, replace the filter, and if it freezes again, call a tech — repeated freezing usually points to a refrigerant leak.

Can I add refrigerant to my AC myself?

No. Refrigerant is federally regulated, requires certification and special gauges, and a low charge means there's a leak that needs to be found and repaired — just topping it off wastes money and harms the environment. This is a licensed-pro job.

Related guides

HouseCue guides are general educational information, not professional inspection, engineering, or contracting advice. Costs vary by market. For safety issues — gas, electrical, structural, or major water — contact a qualified professional.