Home Doctor

HVAC guide

AC running but not cooling: what to check before you call

By the Home Doctor team · Reviewed

Quick answer

When an AC runs but blows warm air, start with the cheap fixes: a clogged air filter, a thermostat set to Heat or Fan On, a tripped breaker for the outdoor unit, or a dirty, blocked outdoor coil. If those check out and the outdoor fan isn't spinning or the lines are iced over, you likely need a technician for a capacitor, a refrigerant leak or another part. A capacitor replacement typically runs $150 to $450, and a refrigerant leak repair $300 to $1,500 or more.

How urgent
Fix soon

Urgent in extreme heat, especially with babies, older adults or anyone with a heat-sensitive medical condition in the house.

DIY or pro
DIY if handy

Filters, thermostat settings, breakers and rinsing the outdoor coil are DIY. Capacitors, refrigerant and compressors are for a licensed HVAC tech.

Stop and call a pro now if

  • Turn off the power at the outdoor disconnect before touching the outdoor unit. Don't open its electrical panel: capacitors hold a charge even with the power off.
  • Burning smell from a vent or the outdoor unit: turn the system off at the thermostat and the breaker, and call a technician.

Common causes and how to tell them apart

  1. 1. Clogged air filter

    How to tell
    Weak airflow from the vents. Sometimes ice on the indoor coil.
    What fixes it
    Replace the filter. Most need changing every 1 to 3 months in cooling season.

  2. 2. Thermostat settings

    How to tell
    Set to Heat or Fan On, the setpoint is above room temperature, or the display is blank.
    What fixes it
    Set it to Cool and Auto, a few degrees below room temperature. Replace the batteries.

  3. 3. Outdoor unit has no power

    How to tell
    The indoor blower runs but the outdoor unit is silent.
    What fixes it
    Check the AC breaker and the disconnect switch next to the outdoor unit.

  4. 4. Dirty or blocked outdoor coil

    How to tell
    Leaves, grass, dryer lint or plants packed around the outdoor unit.
    What fixes it
    With the power off, clear 2 feet around it and rinse the fins gently with a garden hose. No pressure washer.

  5. 5. Frozen coil

    How to tell
    Ice on the copper line at the outdoor unit or on the indoor coil.
    What fixes it
    Set the system to Off and the fan to On for a few hours to thaw. If it ices again, you need a technician.

  6. 6. Failed capacitor or contactor

    How to tell
    The outdoor unit hums but its fan doesn't spin, or it clicks without starting.
    What fixes it
    A technician replaces the part, usually in one visit.

  7. 7. Low refrigerant from a leak

    How to tell
    Long run times, ice on the lines, sometimes hissing.
    What fixes it
    A licensed tech finds and repairs the leak, then recharges. Refrigerant handling requires EPA certification.

Not sure which one you have?

Snap a photo. Home Doctor narrows it down, tells you how urgent it is, and prices the fix.

Diagnose it from a photo

First diagnosis free. No account needed.

What you can safely try first

  1. 1Set the thermostat to Cool and Auto, a few degrees below room temperature.
  2. 2Replace the air filter.
  3. 3Check the breaker and the outdoor disconnect.
  4. 4Clear 2 feet around the outdoor unit.
  5. 5If you see ice, turn cooling off and run the fan to thaw it before anything else.

When to call a pro

  • The outdoor fan doesn't spin, or the unit hums or clicks.
  • Ice comes back after thawing.
  • The breaker trips again.
  • Any burning smell.

What it costs

Typical costs to fix: ac not cooling
JobDIY partsPro, installed
Replace air filter$10–$40—
HVAC diagnostic visit—$75–$200
Replace capacitorNot recommended$150–$450
Replace contactorNot recommended$150–$400
Replace outdoor fan motorNot recommended$300–$700
Find and fix a refrigerant leak, then rechargeNot allowed$300–$1,500+

Ballpark US ranges as of September 27, 2026. Your region, access and the specifics of your home can move these a lot. Get two or three quotes for anything over a few hundred dollars.

What to tell the pro

My central AC runs but blows warm air. The indoor blower [runs / doesn't run], and the outdoor unit [is silent / hums / runs with the fan spinning]. I've replaced the filter and checked the thermostat and breakers. I [see / don't see] ice on the lines. The system is about [age] years old.

Questions people ask

Why is my AC running but not cooling the house?

The most common causes are a clogged filter, wrong thermostat settings, a tripped breaker for the outdoor unit, or a dirty outdoor coil. If those are fine, a failed capacitor or a refrigerant leak is likely, and both need a technician.

Can a dirty filter stop an AC from cooling?

Yes. A clogged filter chokes airflow, which can freeze the indoor coil and stop cooling almost completely. Replacing it is the first thing to try.

Should I keep running my AC if it's not cooling?

No. If there's ice on the lines or the outdoor fan isn't spinning, running it can damage the compressor, the most expensive part. Turn cooling off until it's fixed.

Get an answer for your house

Every house is different. A photo gets you the likely cause, how urgent it is, and a fair price, in about 30 seconds.

Diagnose it from a photo

First diagnosis free. No account needed.

General guidance, not a substitute for a licensed professional who can see the problem in person. If you smell gas, see sparks or smoke, or anyone is hurt, leave the area and call 911.