Window AC Not Cooling: Filter, Frozen Coil, Refrigerant & Compressor

Common Symptoms

• Room temperature doesn't drop despite the unit running
• Air from the vents feels barely cool or room temperature
• Ice visible on the evaporator coil or lines
• Unit runs but compressor doesn't kick on (fan only)
• Compressor cycles on and off every few minutes
• Water dripping from the front of the unit (not normal drain location)

Most Likely Causes

1. 1

   Dirty or Clogged Air Filter (Most Common)

   A clogged filter restricts airflow over the evaporator coil. Without adequate airflow, the coil can't transfer heat efficiently and the unit struggles to cool. In severe cases, restricted airflow causes the coil to freeze over entirely, which further blocks airflow in a self-reinforcing cycle. Cleaning or replacing the filter takes 5 minutes and restores full capacity.

2. 2

   Frozen Evaporator Coil

   A frozen coil can result from a dirty filter, blocked return air, low refrigerant charge, or running the unit with outdoor temperatures below 60°F. Ice on the coil acts as insulation — the unit runs but can't exchange heat. The fix is to turn the unit to 'fan only' mode for 1–2 hours to thaw the coil, then identify and fix the root cause.

3. 3

   Compressor Tripped on Thermal Overload

   Window AC compressors have a built-in thermal overload that shuts the compressor off when it overheats. This often happens when the condenser coils (the outdoor-facing coils) are dirty, restricting heat rejection. The unit will run the fan but you'll hear the compressor click off and on. Clean the condenser coils on the back of the unit.

4. 4

   Low Refrigerant Charge (Leak)

   Refrigerant doesn't deplete naturally — if the charge is low, the system has a leak. Low refrigerant causes the evaporator coil to ice over and the unit to blow barely-cool air. Refrigerant service requires an EPA 608 certified technician and special equipment. A leaking window AC unit is usually a replacement candidate rather than a repair.

5. 5

   Compressor Failure

   A failed compressor means no refrigerant compression and no cooling — the fan runs but the air isn't cooled. Compressor replacement costs more than a new window AC unit in most cases. If the compressor is confirmed failed, replace the unit.

Quick DIY Checks

1. 1Clean or replace the filter: remove the front panel of the window AC unit and locate the air filter (usually a foam or mesh panel). Wash a foam filter in warm water with a little dish soap, rinse thoroughly, and let it dry completely before reinstalling. A paper/fiber filter should be replaced. Reinstall and run the unit.
2. 2Check for a frozen coil: remove the front panel and look at the evaporator coil (the metal fins facing inside). If there is ice on the coil, turn the unit to 'fan only' mode and let it run until the ice fully melts (1–3 hours). Do not chip ice off — let it thaw naturally.
3. 3Clean the condenser coils: look at the back of the unit from outside. The rear-facing fins are the condenser. Use a soft brush or compressed air to clear any dust, leaves, or debris from between the fins. Do not use a pressure washer — the fins are easily bent.

1. 4Verify the unit is sized for the room: window ACs are rated in BTUs. A 5,000 BTU unit in a 500 sq ft room will run continuously without achieving setpoint. A general guideline: 5,000–6,000 BTU for up to 150 sq ft; 8,000–10,000 BTU for up to 350 sq ft; 12,000 BTU for up to 550 sq ft.
2. 5Check that the unit is level or slightly tilted back: window AC units should be installed level or with a slight rearward tilt (1/4 inch) to allow condensate to drain out the back. A unit tilted forward will pool water on the evaporator and ice over.
3. 6Listen for the compressor: when the unit is set to 'cool' and running, you should hear the compressor start — a low hum or thump distinct from the fan noise. If you only hear the fan, the compressor is not engaging. This indicates compressor failure, a tripped thermal overload, or a failed run capacitor.
4. 7Inspect and seal the window installation gaps: with the unit running, run your hand along the accordion side panels, the top rail, and around the exterior of the unit. Any draft of hot outside air entering around the unit drastically reduces cooling. Seal gaps with foam weatherstripping or self-adhesive window seal tape. Also verify the accordion panels are fully extended and locked into the window frame — a partially extended panel leaves a large gap on one side.
5. 8Compare thermostat setpoint to actual room temperature: check that the unit's thermostat is set at least 5–8°F below the current room temperature. Many units won't run the compressor if the setpoint is close to or above the room temp — this is by design. Set the mode to 'Cool' and the temperature to the lowest setting, then wait 3–5 minutes to confirm the compressor starts. If the room is 78°F and the setpoint is 77°F, the compressor may not engage — set it to 70°F for the diagnostic test.
6. 9Test the run capacitor if the compressor won't start: unplug the unit. Remove the back or side panel to access the capacitor — a cylindrical metal can typically located near the compressor. Set your multimeter to capacitance (µF) mode. A healthy run capacitor reads within 10% of its labeled µF value. A reading significantly lower or OL (open circuit) means the capacitor has failed. Replacing a run capacitor ($8–$20) is the most common Intermediate-level repair for a window AC where the fan runs but the compressor does not.
7. 10Look for refrigerant leak signs before calling a technician: refrigerant leaks leave visible evidence — an oily residue or faint rainbow-colored film on the evaporator coil fins or refrigerant tubing, ice forming in an unusual pattern (typically at one spot rather than uniformly across the coil), or a hissing sound when the unit is idle. If you see oily residue near the compressor inlet or suction line joints, the unit has a refrigerant leak that requires EPA 608 certified service. Do not add refrigerant yourself — it is illegal without certification and won't work without sealing the leak.

Repair vs Replace

Recommended Tools & Parts

• Window AC Replacement Air Filter

  Foam or mesh replacement filter for window AC units. Many units use generic sizes — measure your existing filter or search by brand and model number.

  $5–$15

  Buy on Amazon →

• AC Run Capacitor

  Run capacitor for window AC compressor. A failed capacitor is a common cause of compressor not starting. Requires multimeter testing to confirm before replacing.

  $8–$20

  Buy on Amazon →

• AC Coil Fin Comb

  Fin comb straightens bent condenser and evaporator fins on AC units. Bent fins restrict airflow and reduce cooling efficiency significantly.

  $10–$20

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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