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Fix a central AC that will not cool

We'll confirm the thermostat and filter, rule out blocked coils, then isolate the cause—refrigerant, compressor, or fan—or tell you when to call a pro.

Troubleshooting · Home appliances
15–30 min
What you'll need
  • Replacement air filter (if dirty)
  • Garden hose or brush (for outdoor coil)
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Steps

Goal: Confirm thermostat and filter, rule out blocked coils, then isolate the fault.

  • Check the thermostat. It must be set to “Cool” or “AC” and the temperature below room temperature.
  • Good: Thermostat in Cool mode. Proceed to Check thermostat and filter.
  • Bad: Thermostat in Heat or Fan Only—set to Cool and retest.

Check thermostat and filter

Goal: Rule out thermostat setting, power, and filter before assuming a mechanical fault.

  • Check the circuit breaker and disconnect switch. Replace the air filter if dirty.
  • Confirm the outdoor unit runs when the thermostat calls for cooling. You should hear the compressor and fan.
  • Good: Power on, filter clean, outdoor unit runs. Proceed to Outdoor unit not running or No cool air based on symptoms.
  • Bad: Breaker tripped or filter very dirty—fix and retest.

Outdoor unit not running

Goal: Get the outdoor unit to run when cooling is requested.

  • If the outdoor unit does not run, the contactor, capacitor, or thermostat wiring may have failed. Check that power is present at the disconnect.
  • Clean the condenser coil—debris blocks heat transfer and can cause the unit to shut down.
  • Good: Unit runs after power or coil fix.
  • Bad: Unit still does not run—call a technician. The contactor or capacitor may need replacement.

No cool air

Goal: Isolate refrigerant or compressor issues when the unit runs but does not cool.

  • If the unit runs but blows warm or room-temperature air, refrigerant may be low or the compressor may have failed.
  • Ice on the evaporator coil blocks airflow. Turn off the AC, let it thaw, replace the filter. If ice returns quickly, call a technician.
  • Do not attempt to add refrigerant yourself. Refrigerant work requires EPA certification.
  • Good: Technician diagnoses and repairs.
  • Bad: Continuing to run with low refrigerant can damage the compressor—call a pro.

When to get help

Call an HVAC technician if:

  • The thermostat is set correctly and the filter is clean but cooling is weak or absent.
  • The outdoor unit does not run and power checks pass.
  • Ice forms on the indoor coil and returns quickly after thawing.
  • You suspect refrigerant leak or compressor failure.

Refrigerant work must be done by a licensed professional.

Verification

  • Cold air comes from the vents when the thermostat calls for cooling.
  • The outdoor unit runs and the fan spins.
  • Filter is clean and outdoor coil is free of debris.
  • No ice on the indoor coil, no unusual noises or rapid cycling.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm thermostat Check thermostat is set to Cool and below room temperature.
  2. Power and filter Check circuit breaker, disconnect, and air filter.
  3. Coils Clean outdoor coil; check for ice on indoor coil.
  4. Refrigerant / compressor Technician tests refrigerant and compressor—no DIY.
  5. Call a pro Refrigerant, compressor, or contactor—licensed pro only.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Thermostat setting and mode
  • Whether outdoor unit runs
  • Air temperature at vents
  • Filter condition
  • Ice on indoor coil?
  • Steps already tried

Is the thermostat set to Cool and below room temperature?

Thermostat must be in cooling mode.

Check the thermostat. Set to "Cool" or "AC." Set temperature below room temperature. Wait a few minutes. Yes: proceed to power and filter. No: set correctly and retest.

Yes No

You can change your answer later.

Set thermostat to Cool and retest

Set thermostat to Cool. Set temperature below room temperature. Wait 5 minutes. Check if cold air comes from vents. If yes, problem solved. If no, proceed to power and filter.

Done

Is power on and the filter clean?

Breaker, disconnect, and filter affect operation.

Check circuit breaker and disconnect switch. Replace dirty air filter. Good: power on, filter clean. Bad: fix and retest.

Yes No

You can change your answer later.

Fix power and filter

Reset breaker. Turn on disconnect. Replace filter. Retest. If cooling returns, done. If not, proceed to outdoor unit check.

Done

Does the outdoor unit run when cooling is requested?

The compressor and fan should run when the thermostat calls for cool.

When thermostat calls for cool, the outdoor unit should run. Yes: unit runs—check if air is cold. No: unit does not run—check contactor, capacitor, or call a pro.

Yes No

You can change your answer later.

Is cold air coming from the vents?

If unit runs but air is warm, refrigerant or compressor may be the issue.

Feel the air at the vents. Cold: cooling works—may have been thermostat or filter. Warm/room temp: refrigerant may be low, or compressor failed. Technician must check—no DIY.

Yes No

You can change your answer later.

Cooling restored

Cooling is working. Check that the condenser coil is clean for best performance.

Done

Call a technician

Call an HVAC technician for: outdoor unit not running (contactor, capacitor), weak or no cooling (refrigerant, compressor). Refrigerant work requires a licensed professional.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a central AC run but not cool?
Common causes: thermostat set to heat or fan-only, dirty filter, blocked outdoor coil, low refrigerant, or failed compressor. Check thermostat and filter first, then have a technician test refrigerant and compressor.
Can I fix a central AC that will not cool myself?
You can check the thermostat, replace the filter, and clean the outdoor coil. Refrigerant and compressor work require a licensed HVAC technician. Do not attempt to add refrigerant yourself—it is regulated and requires certification.
When should I call a technician for a central AC that will not cool?
Call if the thermostat is set correctly and the filter is clean but cooling is weak or absent. Call for refrigerant leaks, compressor failure, or if the outdoor unit does not run and power checks pass. Refrigerant work must be done by a licensed professional.

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