* Blend Door Actuator (Driver's Side): This is the most probable cause. The blend door actuator is a small motor that controls the mix of hot and cold air directed to each vent. If the driver's side actuator is malfunctioning or stuck, it can't properly blend the cold air, resulting in only hot air coming out. This is especially likely given the disparity between driver's and passenger sides.
* Blend Door Cable (if equipped): Some vehicles, though less common now, use cables instead of actuators. If a cable is broken or frayed, it won't be able to properly position the blend door.
* Blend Door itself: The blend door could be broken or jammed, preventing proper movement regardless of the actuator's function.
* Mode Door Actuator (less likely but possible): This controls the airflow direction (floor, vents, defrost). A malfunctioning mode door actuator *could* indirectly affect temperature distribution if it's preventing proper air circulation to the driver's side.
* Vacuum Leaks (less likely): Some older HVAC systems use vacuum lines to control the blend doors. Leaks in these lines could lead to improper operation. This is less likely given the one-sided nature of the problem.
* Clogged vents: Although less likely to cause a temperature difference this dramatic, a severely clogged vent on the driver's side *could* contribute to reduced airflow and thus a warmer temperature.
Troubleshooting Steps:
1. Check the HVAC controls: Make sure the temperature is set low on the driver's side. If it's set to "hot," the actuator is likely working but trying to provide hot air.
2. Listen for the actuator: When you change the temperature setting, you should hear a faint clicking or whirring sound from the blend door actuator (located inside the dashboard – access varies by vehicle year). If you don't hear anything on the driver's side, it's highly suspect.
3. Visual Inspection (difficult): Accessing the blend door actuator and blend door requires partial dashboard disassembly. This is an advanced repair and usually best left to a professional. A visual check *might* reveal a broken cable (if applicable) or obvious mechanical issues.
4. Professional Diagnosis: The most efficient approach is to have a qualified mechanic diagnose the problem. They have the tools and expertise to pinpoint the faulty component. They can also properly access the area behind the dashboard and test the actuator with diagnostic equipment.
Important Note: Since your compressor lines are cold, your refrigerant level is likely fine. Focus your attention on the blend door system, as that's the most probable cause of this one-sided warm-air issue. Don't jump to conclusions about refrigerant leaks or compressor failure until you've addressed the more likely blend door problem.
