When your window air conditioner’s fan runs but the compressor won’t turn on, you’re left with circulating warm air instead of the cooling relief you need. This common but frustrating problem can stem from various issues ranging from simple power problems to complex component failures. A non-functioning compressor is typically indicated by the unit blowing uncooled air, making unusual noises, or failing to start altogether. Understanding the potential causes and troubleshooting steps can help you diagnose the problem and determine whether it’s a simple DIY fix or a situation requiring professional assistance.
The compressor serves as the heart of your air conditioning system. This vital component circulates refrigerant through the system, enabling the heat exchange process that cools your room. When functioning properly, the compressor pressurizes the refrigerant, which then travels through coils where it absorbs heat from indoor air.
Window air conditioners typically have a two-part operation. The fan circulates air through the unit, while the compressor handles the actual cooling process. When only the fan runs but the compressor doesn’t engage, the unit might still blow air, but it won’t cool your space.
Signs that your compressor isn’t turning on include:
- The unit blows room-temperature air
- You can hear the fan but not the distinctive hum of the compressor
- The unit starts briefly then stops
- The air conditioner trips your circuit breaker when attempting to start
Common Causes of Window AC Compressor Failure
Several issues can prevent your window AC compressor from turning on. Understanding these common causes can help you pinpoint the problem.
Power Supply Issues
Inadequate power supply is one of the most common reasons a compressor fails to start. Window air conditioners, especially larger models, require significant electrical current. Problems may include:
- Tripped circuit breakers or blown fuses
- Loose or damaged power cords
- Voltage fluctuations in your home’s electrical system
- Using an extension cord that can’t handle the amperage
Thermostat Problems
The thermostat controls when your compressor activates. If it’s malfunctioning or incorrectly set, your compressor won’t receive the signal to turn on. Possible thermostat issues include:
- Incorrect temperature settings
- Faulty temperature sensors
- Internal wiring problems
- Calibration errors
Start Capacitor Failure
The start capacitor provides the electrical boost needed for the compressor to begin running. When this component fails, the compressor may not have enough power to start. Signs of capacitor failure include:
- Clicking sounds when the unit tries to start
- Compressor attempts to start but can’t
- Bulging or leaking on the capacitor itself
- Burning smell from the unit
Overload Protector Issues
The overload protector safeguards your compressor from electrical overloads and overheating. If it’s defective or repeatedly triggered, it may prevent the compressor from running.
Control Board Malfunctions
Modern window air conditioners utilize electronic control boards to manage operations. Damaged circuit boards can disrupt communication between components, preventing the compressor from activating. Water damage, power surges, or simple component failure can all affect control board function.
Dirty or Clogged Components
Accumulated dirt and debris can impede airflow and cause your unit to overheat. When this happens, safety mechanisms prevent the compressor from running to avoid damage.
DIY Troubleshooting Steps
Before calling a professional, try these troubleshooting steps to potentially resolve the issue yourself.
Check Power Supply and Connections
- Inspect the power cord for damage or loose connections
- Ensure the unit is plugged directly into a wall outlet, not an extension cord
- Check your circuit breaker panel for tripped breakers
- Test the outlet with another appliance to confirm it’s working
Verify Thermostat Settings
Ensure your thermostat is set correctly:
- Set the mode to “Cool” (not “Fan”)
- Set the temperature several degrees below room temperature
- Check that sensors aren’t blocked by furniture or curtains
- Try resetting the thermostat if your model allows it
Clean Filters and Coils
Dirty filters and coils restrict airflow, causing the unit to overheat and shut down. Regular cleaning is essential:
- Remove and clean or replace the air filter
- Gently clean the evaporator coils (inside coils) with a soft brush or coil cleaner
- Clean the condenser coils (outside coils) to remove dirt and debris
- Clear any obstructions from air intake and output vents
Reset the Unit
Sometimes a simple reset can resolve electronic glitches:
- Unplug the unit for at least 30 minutes
- During this time, press and hold the power button to discharge residual electricity
- Plug the unit back in and attempt normal operation
Check for Ice Buildup
Ice accumulation on the evaporator coils can prevent proper operation. If you see ice:
- Turn off cooling and run the fan only to melt the ice
- Ensure proper airflow isn’t blocked
- Check for dirty filters that might restrict airflow
Advanced Troubleshooting
The following steps involve working with electrical components and should only be attempted if you’re comfortable with such tasks. Always disconnect power before inspecting internal components.
Testing the Start Capacitor
A faulty start capacitor is a common cause of compressor issues:
- Disconnect power and remove the unit’s cabinet following manufacturer instructions
- Locate the start capacitor (typically a cylindrical component near the compressor)
- Check for visible damage such as bulging, leaking, or burn marks
- Use a multimeter to test capacitance if you have the knowledge and equipment
Examining the Overload Protector
The overload protector can fail or become overly sensitive:
- Locate the overload protector (usually attached to the compressor or nearby)
- Inspect for visible damage
- Check for continuity using a multimeter if you have the skills
Inspecting the Control Board
Control board issues require careful visual inspection:
- Look for burnt components, corrosion, or loose connections
- Check for blown fuses on the board
- Inspect for signs of water damage or electrical arcing
| Component | Signs of Failure | Diagnostic Steps | Typical Repair Cost |
|---|---|---|---|
| Start Capacitor | Clicking sound, bulging appearance, burning smell | Visual inspection, multimeter testing | $10-$40 (DIY) / $100-$200 (professional) |
| Overload Protector | Compressor shuts off quickly, won’t start | Continuity testing, visual inspection | $15-$50 (DIY) / $100-$250 (professional) |
| Control Board | Erratic operation, no response to controls | Visual inspection, voltage testing | $50-$200 (DIY) / $150-$350 (professional) |
| Compressor | Humming without starting, complete silence | Ohm testing, professional diagnosis | $200-$400 (part only) / $400-$800 (installed) |
When to Call a Professional
While many issues can be resolved through DIY troubleshooting, some problems require professional expertise. Call a technician if you encounter any of these situations:
- Electrical burning smells or visible smoke
- Refrigerant leaks (indicated by oil spots or hissing sounds)
- Compressor that’s hot to the touch or making unusual noises
- Water leaking inside the room beyond the normal drainage
- Multiple failed attempts at DIY repairs
Repair vs. Replacement Considerations
Sometimes replacing the unit makes more financial sense than repairing it. Consider these factors:
| Factor | Favor Repair | Favor Replacement |
|---|---|---|
| Unit Age | Less than 5 years old | More than 8 years old |
| Repair Cost | Less than 50% of new unit cost | More than 50% of new unit cost |
| Efficiency | Energy-efficient model | Older, inefficient model |
| Repair History | First major repair | Multiple previous repairs |
Preventative Maintenance to Avoid Future Problems
Regular maintenance can prevent many compressor issues before they occur. Implementing a consistent maintenance routine significantly extends your air conditioner’s lifespan.
Routine Cleaning
- Clean or replace air filters every 30 days during heavy use
- Clean the front grille and air vents monthly
- Clean evaporator and condenser coils at the beginning of each cooling season
- Keep the area around the outdoor portion of the unit clear of debris
Off-Season Storage
Proper storage during off-seasons prevents premature component failure:
- Clean the unit thoroughly before storage
- Run the unit in fan-only mode for a few hours to dry internal components
- Cover the outdoor portion or remove the entire unit during winter
- Store in an upright position in a dry location if removed
Start-of-Season Preparations
Before the cooling season begins:
- Inspect the power cord and plug for damage
- Clean or replace the air filter
- Check that drainage channels are unobstructed
- Ensure the unit is properly secured in the window
- Run a test cycle to identify any issues before hot weather arrives
Environmental Factors Affecting Compressor Performance
External conditions can impact your air conditioner’s operation and should be considered when troubleshooting:
- Extreme heat: When outdoor temperatures exceed 95°F (35°C), compressors work harder and are more prone to overheating
- Direct sunlight: Units exposed to direct sun experience higher operating temperatures
- Poor installation: Improperly leveled units can cause compressor oil distribution problems
- Voltage fluctuations: Power brownouts or surges can damage electronic components
Consider installing awnings or external shades over window units exposed to direct sunlight, and use a surge protector for your air conditioner to prevent electrical damage during storms or power fluctuations.