AC Running But Not Cooling? 7 Causes and What Each Costs to Fix
An AC that runs constantly but never cools is usually one of seven problems — and they range from a $20 filter to a $2,500 compressor. Work through them in this order before anyone talks you into a new system.
Start with the two free checks
Confirm the thermostat is on Cool and set below room temperature, then pull the air filter. A filter clogged with dust chokes airflow so badly the evaporator coil can freeze into a block of ice — the AC runs, but almost no air moves. If the filter is gray and matted, replace it, switch the system to Fan only for 2–3 hours to thaw the coil, then try cooling again.
The 7 causes, cheapest to most expensive
| Problem | What you'll notice | Typical cost |
|---|---|---|
| Dirty air filter | Weak airflow, ice on refrigerant lines | $10 – $40 DIY |
| Tripped breaker / outdoor unit off | Indoor fan runs, outdoor unit silent | $0 – $150 |
| Dirty condenser coil | Outdoor unit hot, runs nonstop | $100 – $400 (cleaning) |
| Failed capacitor | Outdoor fan hums or clicks but won't spin | $150 – $400 |
| Refrigerant leak | Ice on lines, hissing, gradual decline | $400 – $1,600 |
| Failed condenser fan motor | Outdoor unit trips on overheating | $400 – $700 |
| Failed compressor | Loud clunk, breaker trips, no cooling | $1,500 – $2,800 |
The refrigerant trap to avoid
If a tech proposes “topping off” refrigerant without finding the leak, push back. Refrigerant doesn't get used up — low charge always means a leak, and a top-off without a repair is a recurring bill. On older R-410A systems, repeated top-offs plus a leak search can quickly cost more than the repair itself; get the leak located and priced first.
When not cooling means replace
A compressor failure on a system 12+ years old is usually the end of the road — you'd be putting a third of a new system's price into the oldest part of it. See our repair-or-replace framework and the full HVAC cost guide, or run your numbers in the HVAC cost calculator.
Get at least two quotes for any repair over $500. Diagnosis fees ($75–$150) are usually credited toward the repair if you hire that company.
Frequently asked questions
Why is my AC running but not cooling the house?
The most common causes are a clogged filter, a frozen evaporator coil, a dirty condenser coil, a failed capacitor, or low refrigerant from a leak. Check the filter and breaker first — they're free — then have a tech test the capacitor and refrigerant pressure.
How much does it cost to fix an AC that's not cooling?
Simple fixes run $100–$400 (coil cleaning, capacitor). Refrigerant leak repairs typically run $400–$1,600 depending on location, and compressor replacement $1,500–$2,800. On systems over 12 years old, a compressor failure usually justifies replacement instead.
Is it OK to run the AC if it's not cooling?
Briefly, but if the refrigerant lines are iced over or the outdoor unit is cycling on and off rapidly, shut it down. Running a frozen or low-charge system can destroy the compressor — the most expensive part.