How to Fix a Window Air Conditioner That's Not Cooling (2026)
A window AC that runs but doesn't cool is usually a clogged filter, frozen coil, or dirty condenser — not a refrigerant failure. This guide covers the common causes and fixes for window air conditioners.
Window AC not cooling: (1) Check the filter first — a clogged filter starves the unit of airflow and causes the evaporator coil to freeze over. Remove, clean, and reinstall the filter. If the coil is frozen: turn the unit off for 2 hours with just the fan running to thaw it, then clean the filter before running on cool again. (2) Check the mode: confirm it is set to COOL, not just FAN. (3) Dirty condenser coils on the outside reduce heat rejection capacity — clean the outer fins with a garden hose spray. (4) Room size vs. BTU: a unit that is too small for the room it is cooling will run constantly without ever reaching the setpoint.
Frequently Asked Questions
Why is my window AC running but not cooling the room?
Window AC running but not cooling — causes: (1) Clogged air filter — this is the most common cause. A dirty filter blocks airflow across the evaporator coil. The coil gets too cold and ices over, which then blocks airflow further. The unit blows air but it's not cold because warm air can't reach the coil. Fix: remove and clean the filter (most are washable — rinse under warm water, let dry fully before reinstalling). (2) Frozen evaporator coil — related to the above. If you see frost or ice on the front grille or hear less airflow than normal, the coil is frozen. Turn the unit to FAN ONLY for 2 hours to thaw, then fix the root cause (dirty filter or low refrigerant). (3) Thermostat set too high or in fan-only mode — confirm the temperature setting and that the unit is in COOL mode, not just FAN. (4) Dirty condenser coils (outside) — the back of the unit expels heat. If the outdoor fins are clogged with dirt, cottonwood seeds, or debris, the unit cannot reject heat and cooling capacity drops. Clean the exterior fins. (5) Unit too small for the room — a 5,000 BTU unit can handle about 150 square feet; 8,000 BTU handles about 350 square feet. If the unit cannot keep up, it runs continuously without reaching setpoint — this is a sizing problem, not a malfunction. (6) Refrigerant leak — if the coil never ices and the filter is clean but the unit is not cooling: refrigerant loss is possible. Window AC refrigerant service requires an EPA-certified technician; the cost often exceeds replacement value for small units.
How do I fix a frozen window AC coil?
Fixing a frozen window AC evaporator coil: (1) Recognize a frozen coil: the front grille may have visible frost or ice, airflow from the unit will be noticeably reduced, and the air coming out may feel warmer than expected. (2) Thaw procedure: turn the AC to FAN ONLY mode (or turn it completely off — do not run in COOL mode while frozen). Let the fan run for 2 hours. The coil will thaw and drain. Place towels under the unit inside in case water drips. Do not use a heat gun or hair dryer to accelerate thawing — this can damage the fins. (3) Find and fix the cause: a coil freezes because airflow is insufficient (dirty filter or blocked vents) or because ambient temperature is below 60°F. Clean the filter thoroughly. Check that no furniture is blocking the front grille. Window AC units should not be run in cool mode when outdoor temperatures are below 60°F. (4) After thawing: install the clean dry filter. Run the unit on FAN for 10 minutes, then switch to COOL. Monitor for re-freezing. If the coil freezes again within hours despite a clean filter: the refrigerant charge may be low (requires professional service).
How do I clean a window air conditioner?
Window AC cleaning procedure: (1) Turn off and unplug the unit. (2) Remove and wash the filter: slide out the front filter panel. Rinse under warm water, let dry completely (at least 1 hour), then reinstall. Do this monthly during heavy use. (3) Clean the evaporator fins (front coil): these are the thin metal fins behind the filter. Use a fin comb or a soft brush to straighten any bent fins and remove dust. Do not bend or damage the fins. Spray with coil cleaner foam (Frost King AC coil cleaner, $8–$12) which foams up and drains out. (4) Clean the condenser fins (back/outside coil): with the unit accessible from outdoors, use a garden hose to spray from the inside out through the fins to push debris outward. Do not use a pressure washer — regular hose pressure is enough. Allow to dry before operating. (5) Clean the drain pan: the drip pan at the bottom collects condensate. Wipe out debris and any standing water. Add a few drops of bleach to prevent algae growth. (6) Annual deep clean: once per season, slide the chassis out of the window sleeve (the unit slides forward once the side panels are retracted) for a complete clean of both coil faces. Reinstall and re-seal the gaps around the unit with foam weatherstrip.
My window AC is making loud noises. What do I check?
Window AC noise diagnosis: (1) Rattling: loose mounting in the window frame. The unit should be fully supported by the windowsill and secured with the side accordion panels. Check that the unit is level and snug. A unit that tilts back too far (toward the outside) also drips water incorrectly. (2) Grinding or squealing from the blower: the blower fan (indoor side) may have accumulated debris on the fan blades, causing an imbalance and bearing wear. Unplug, remove the front panel, and clean the fan wheel. A bearing that is truly worn makes a constant squealing sound and requires motor replacement — often not economical on older units. (3) Clicking at startup: normal — the compressor engaging. Clicking during operation (not just at start): can indicate the relay or a failing compressor. (4) Water dripping inside: a properly installed window AC drips water outside. Dripping inside means the unit is tilted slightly toward the inside (should tilt 1/4 inch toward the outside), or the drain pan overflow port is blocked. Check the installation angle. (5) Banging or clanking: the fan blade is hitting the fan housing or a foreign object (leaf, debris) has entered through the outdoor grille. Unplug and inspect both the fan and the outdoor grille.
How do I know when to replace a window air conditioner?
Window AC replacement decision: (1) Age and efficiency: most window AC units last 8–12 years. Units over 10 years old typically consume 20–30% more electricity than current Energy Star models — replacement can pay for itself in 2–3 years in reduced electricity costs. (2) Refrigerant service: if a technician diagnoses a refrigerant leak, repair cost ($150–$300) usually exceeds the replacement cost of a 5,000–8,000 BTU unit ($150–$250). Only worthwhile on large or high-end units. (3) Compressor failure: a compressor that will not run means the AC cannot cool at all. Compressor replacement is not DIY and typically costs $200–$400 in labor and parts — almost always replace the unit instead. (4) Cooling capacity: if the unit is undersized for the room and has been running at full capacity for years, it will wear out faster. Replacing with a correctly-sized unit (BTU matched to room square footage) provides better comfort and longer unit life. (5) Mold inside the unit: mold that has infiltrated the evaporator fins or blower wheel and cannot be cleaned thoroughly is a health concern. Replace the unit — a thorough clean requires full disassembly that is not practical on most residential units.
Window AC not cooling: (1) Check the filter first — a clogged filter starves the unit of airflow and causes the evaporator coil to freeze over. Remove, clean, and reinstall the filter.
Start with the filter — most window AC cooling failures are a clogged filter causing the coil to freeze, not a refrigerant problem.
What you need
- Clean or replacement air filter
- Fin comb or soft brush
- Coil cleaner foam (for deep cleaning)
- Garden hose (for outdoor coil)
Step 1: Clean or replace the filter
Slide out the filter, rinse under warm water, and let dry completely. If the unit has been running poorly, the coil may already be frozen.
Step 2: Thaw a frozen coil
If airflow feels weak or you see frost: set to FAN ONLY for 2 hours. Towels under the unit catch drips. Do not run on COOL until fully thawed.
Step 3: Clean the outdoor condenser fins
From outside, spray the condenser fins with a garden hose (inside-out direction) to clear debris. Let dry before running.
Step 4: Test and monitor
Run the unit on COOL with a clean dry filter. If the coil re-freezes within a few hours: call a technician to check refrigerant charge.
Most window AC problems are fixed by cleaning the air filter, straightening bent condenser fins, or clearing a clogged drain. If the unit runs but doesn’t cool: dirty evaporator coils or low refrigerant (needs a tech). If it won’t turn on: check the outlet with another appliance and test the reset button on the power cord. If it leaks water inside: the unit is tilted too far forward — it should pitch slightly back so condensate drains out. Most repairs take under an hour with no tools.
Related guides
- How to Fix an Air Conditioner Not Cooling — central AC cooling failure diagnosis
- How to Change a Furnace Filter — HVAC filter replacement
- How to Fix a Noisy HVAC — diagnosing HVAC noise
- Clean or replace the air filter
The most common cause of a window AC that runs but doesn't cool is a clogged air filter. Pull the unit's front grille off (it usually snaps or slides out). Remove the foam or mesh filter behind it. If the filter is gray and matted: wash it with warm soapy water, rinse, and let it dry completely before reinstalling — or replace it. A clogged filter restricts airflow so severely that the evaporator coil freezes over, blocking all cooling. Run the unit with just the fan (no cooling) for 30–60 minutes after cleaning to thaw any ice buildup before testing cooling again.
- Clean the evaporator and condenser coils
The evaporator coils (inside-facing, usually aluminum fins behind the filter) collect dust that insulates them and reduces heat transfer. With the unit unplugged: use a soft brush or vacuum with a brush attachment to gently clean the fins. Do not bend the fins. For the condenser coils (the outside-facing section with larger fins): spray with a coil cleaner or rinse from the outside with a garden hose on gentle pressure. Dirty condenser coils prevent the unit from rejecting heat outdoors, causing the compressor to overheat and shut off.
- Check for a frozen coil
If the unit is blowing but the air is not cold and you see ice on the coils or frost on the unit body: the coil is frozen. Turn the unit to fan-only mode or off, and let it defrost completely (1–2 hours). Then check the cause: clogged filter (most common), blocked airflow from furniture or curtains too close to the unit, or outdoor temperatures below 60°F (window ACs should not run in cooling mode when it is below 60°F outside). Fix the root cause before running cooling mode again.
- Confirm the mode and thermostat setting
Verify the unit is in COOL mode (not FAN or DRY/DEHUMIDIFY mode). Set the thermostat several degrees below the current room temperature. Give the compressor 2–3 minutes to start after switching to cool mode — a short delay before the compressor engages is normal. If the compressor starts (you should hear a louder hum and feel noticeably colder air from the vents within 5 minutes) but the room doesn't cool: check for air leaks around the installation — warm outside air bypassing the unit reduces cooling efficiency significantly.
- Inspect the installation seal
A window AC that runs but struggles to cool the room may be losing efficiency through gaps around the unit. Inspect the foam side panels and the seal between the unit and the window frame — if warm outside air is entering around the sides, the unit is fighting a losing battle. Foam weatherstrip tape ($5–$8) fills gaps between the accordion panels and the window sashes. Apply to all gaps on the interior side. Also confirm the unit is tilted slightly outward (1/4 inch) so condensate drains outside rather than collecting internally.
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