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Fix a window AC that will not cool

We'll check the filter, thermostat, seal, fan, condenser coils, and refrigerant—or tell you when to call a pro.

Troubleshooting · Home appliances
15–45 min
What you'll need
  • Soft brush and coil cleaner (or mild soap)
  • Replacement filter (if not washable)
  • Multimeter (optional, for capacitor tests)
Step 1 of 6

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Steps

Goal: Confirm the symptom, rule out filter and thermostat, then isolate the cooling fault.

  • Turn the AC on Cool mode and set the temperature below room temperature. Wait five minutes. Feel the air from the vents.
  • Good: The fan runs but the air is warm—cooling system has failed. Proceed to Check filter and thermostat.
  • Bad: The fan does not run—different problem (power, motor). Check power and fan motor first.

Check filter and thermostat

Goal: Rule out simple causes before opening the unit.

  • Remove the front grille or filter panel. Pull out the air filter. Clean it (washable) or replace it if dirty.
  • Confirm the mode is set to Cool and the temperature is at least 5°F below room temperature.
  • Check the seal between the unit and the window frame. If gaps are visible, adjust the side panels or add foam weatherstrip.
  • Good: Filter is clean, thermostat is correct, and seal is snug. Proceed to Clean condenser coils.
  • Bad: Filter was dirty, thermostat was wrong, or seal had gaps—fix and retest. If still no cooling, proceed to coil cleaning.

Clean condenser coils

Goal: Remove dirt from the condenser coils so heat can be rejected.

  • Unplug the unit. Remove the outer casing if needed per your model. Locate the condenser coils on the outside.
  • Use a soft brush and coil cleaner (or mild soap and water) to clean the coils. Rinse gently. Let dry before reassembling.
  • Check the evaporator coils (inside, behind the filter). If frozen, turn off and let thaw. Clean if dusty.
  • Good: Coils are clean. Proceed to Fan and compressor.
  • Bad: Coils were dirty—clean thoroughly and retest.

Fan and compressor

Goal: Confirm the condenser fan and compressor run when cooling is requested.

  • Look at the rear of the unit. The condenser fan should spin when the compressor runs. Listen for the compressor hum.
  • If the fan does not spin, the motor or capacitor may have failed—call a technician.
  • If the compressor does not run, the capacitor, overload, or compressor may have failed—call a technician.
  • Good: Both run. If coils are clean and it still does not cool, low refrigerant or other fault—call a technician.
  • Bad: Fan or compressor not running—call a technician.

When to get help

Call an HVAC technician if:

  • The filter and coils are clean, the thermostat is set correctly, and the unit still blows warm air.
  • You hear hissing (refrigerant leak).
  • The compressor does not run.
  • You are not comfortable with electrical work.

Refrigerant work requires EPA certification. Do not attempt to add refrigerant yourself.

Verification

  • The unit blows cold air within five minutes of starting Cool mode.
  • No ice on the evaporator coils.
  • Filter is clean and condenser coils are free of debris.
  • No hissing or unusual noises.

Escalation ladder

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

  1. Confirm symptom Verify the unit runs but blows warm air; rule out a different problem.
  2. Filter, thermostat, and seal Clean or replace the filter; confirm Cool mode and low temperature setting; check the window seal.
  3. Condenser coils Clean the condenser coils on the outside of the unit.
  4. Fan and compressor Confirm condenser fan spins; listen for compressor operation.
  5. Call a pro Refrigerant leak, compressor not running, or still no cooling—call an HVAC technician.

What to capture if you need help

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

  • Filter condition (clean or dirty)
  • Thermostat mode and temperature setting
  • Window seal condition (gaps or snug)
  • Whether condenser fan and compressor run
  • Coil condition (clean or dirty)
  • Steps already tried

Does the unit run but blow warm air?

Turn on Cool mode and set temperature below room temp. Wait five minutes. Feel the vent air.

Turn the AC on Cool mode and set the temperature below room temperature. Wait five minutes. Feel the air from the vents. Good: fan runs but air is warm—cooling fault. Bad: fan does not run—different problem (power, motor); check power first.

Yes No

You can change your answer later.

Is the filter clean, thermostat set to Cool, and seal snug?

A dirty filter blocks airflow. Thermostat must be in Cool mode with temp below room temp. A poor seal lets warm air leak in.

Remove and inspect the air filter. Clean or replace if dirty. Confirm mode is Cool and temperature is below room temp. Check the seal between the unit and the window frame—add weatherstrip if gaps exist. Good: filter clean, thermostat correct, seal snug. Bad: filter dirty, thermostat wrong, or seal has gaps—fix first.

Yes No

You can change your answer later.

Are the condenser coils clean?

Dirty condenser coils block heat rejection. Clean with soft brush and coil cleaner.

Unplug the unit. Clean the condenser coils on the outside with a soft brush and coil cleaner. Rinse and dry. Good: coils clean. Bad: still dirty—clean thoroughly.

Yes No

You can change your answer later.

Does the condenser fan spin and compressor run?

Condenser fan rejects heat. Compressor circulates refrigerant. Both must run for cooling.

Check the rear of the unit. The condenser fan should spin when cooling. Listen for the compressor hum. Fan not spinning: motor or capacitor—call a pro. Compressor not running: capacitor or compressor—call a pro. Both run: may be low refrigerant—call a pro.

Yes No

You can change your answer later.

No hissing or oil stains?

Hissing or oil near lines can indicate a refrigerant leak. Do not add refrigerant yourself.

Check for hissing, bubbling, or oil stains near refrigerant lines. Hissing or oil: refrigerant leak—call a technician. No signs: if coils are clean and both run but still no cooling, low refrigerant or other fault—call a technician.
Question

No hissing or oil stains?

Done

Call a technician

Call an HVAC technician if: the filter and coils are clean, thermostat is correct, and it still blows warm; you hear hissing (refrigerant leak); the compressor does not run; or you are not comfortable with electrical work. Refrigerant work requires EPA certification.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a window AC run but not cool?
Common causes: dirty air filter blocking airflow, thermostat set wrong, dirty condenser coils, low refrigerant, or failed compressor. Check filter and thermostat first, then clean the coils. Refrigerant and compressor issues need a technician.
Can I fix a window AC that will not cool myself?
Yes, for filter cleaning, thermostat checks, and coil cleaning. Refrigerant leaks and compressor repairs require EPA certification—call a technician. Do not attempt to add refrigerant yourself.
When should I call a technician for a window AC?
Call a technician if the coils are clean and the filter is new but it still blows warm air, you hear hissing (refrigerant leak), or the compressor does not run. Refrigerant work is regulated and requires certification.

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