Your window AC unit’s fan is blowing air, but you’re not getting any cooling because the compressor won’t start. This means your AC can’t produce cold air, leaving you with an expensive fan during the hottest days of 2025.
Step-by-Step Fixes
Step 1: Check Your Thermostat Settings
Start with the simplest fix. Make sure your thermostat is set to “Cool” mode, not just “Fan” mode. The temperature setting needs to be at least 5 degrees below the current room temperature. Many window units have a digital display showing both settings. If your room is 75°F, set the thermostat to 70°F or lower to trigger the compressor.
Step 2: Reset the Circuit Breaker
Head to your electrical panel and look for the breaker controlling your AC unit. Window air conditioners typically need their own dedicated 15-amp or 20-amp circuit. Flip the breaker completely OFF, wait 30 seconds, then flip it back ON. This resets the unit’s internal protection circuits and often resolves compressor startup issues.
Step 3: Clean or Replace the Air Filter
Pull out the front filter panel on your window AC. If the filter looks gray or clogged with dust, the unit might be protecting itself from overheating. Rinse the filter under warm water, let it dry completely, then reinstall. Extremely dirty filters can cause the evaporator coils to freeze, which automatically shuts down the compressor while keeping the fan running.
Step 4: Check the Power Cord and Outlet
Inspect your AC’s power cord for any damage, burns, or melted spots. Test the outlet with another appliance to ensure it’s providing full power. Window AC compressors need significant startup current, and a weak outlet connection might allow the fan to run but not provide enough power for the compressor. Never use an extension cord with window AC units.
Step 5: Wait for the Compressor Delay Timer
Modern window AC units have a built-in 3-5 minute delay before the compressor starts. This protects the compressor from damage if power is cycled quickly. After turning on your unit or changing settings, wait at least 5 minutes before assuming there’s a problem. You should hear a distinct clicking sound when the compressor engages.
Step 6: Inspect the Capacitor (Advanced)
If you’re comfortable with basic electrical work, unplug the unit and remove the outer casing. Look for a cylindrical component (the capacitor) near the compressor. Check for bulging, leaking, or burn marks. A failed capacitor is ideal for preventing compressor startup while allowing the fan to operate normally. This repair is best handled by professionals due to electrical hazards.
Likely Causes
Cause #1: Failed Start Capacitor
The start capacitor provides the electrical boost your compressor needs to begin running. When it fails, the fan motor (which has its own capacitor) continues working fine. You might hear a humming sound when the unit tries to start the compressor. Check for this by listening carefully when the unit should be cooling. A multimeter can test capacitor function, but replacement requires technical knowledge and costs around $20-50 for the part.
Cause #2: Low Refrigerant or Sealed System Problem
Window AC units from major brands like LG, Frigidaire, or GE are sealed systems not recommended for DIY refrigerant work. Low refrigerant causes the low-pressure safety switch to prevent compressor operation. Signs include ice formation on the indoor coils or copper lines. Unlike central systems, window units are rarely worth recharging due to labor costs exceeding replacement value.
Cause #3: Overheating Compressor or Thermal Overload
Compressors have internal thermal protection that shuts them down when overheated. This commonly happens in units over 5 years old or those running continuously in extreme heat. The fan keeps running to help cool the system. Feel the sides of your unit – if they’re extremely hot, turn it off for 2-3 hours to cool completely. Persistent overheating indicates the compressor is failing and replacement is needed.
When to Call Expert Help
Contact a professional HVAC technician when you hear grinding noises from the compressor area, see burn marks on electrical components, or if the unit trips your circuit breaker repeatedly. Window AC repair becomes uneconomical when labor costs exceed $150-200, as new units start around $200-300 in 2025. Most warranty claims require professional diagnosis, so check your documentation before attempting repairs on units under warranty.
Consider replacement instead of repair for window AC units over 8 years old. Energy efficiency improvements in newer models often offset replacement costs through lower electric bills. Professional technicians can safely handle refrigerant recovery and disposal if replacement is necessary.
Copy-Paste Prompt for AI Help
“My window AC unit fan is working but the compressor won’t turn on. The unit is [age] years old, model [insert model number]. I’ve already tried: [list what you’ve done]. The room temperature is [temp] and thermostat is set to [setting]. When I turn it on, I hear [describe sounds]. Should I attempt repair or replace this [BTU size] unit?”