Here's a breakdown of why:
* Blend Doors: These are internal flaps within the HVAC system that control the mix of hot and cold air. They direct airflow to different vents (floor, dash, defrost, side vents).
* Blend Door Actuator: This is a small motor that moves the blend doors. If the actuator for the side vents is malfunctioning (stuck open, partially open, or not receiving the correct signal), it would cause hot air to continuously flow regardless of the temperature setting. The passenger side might be fine because it's on a separate blend door circuit.
* Vacuum System (Less likely, but possible): Older vehicles sometimes used a vacuum system to control blend doors. A leak or malfunction in the vacuum lines could prevent the correct operation of the blend doors, leading to the imbalance in air temperature.
* Wiring Issues: A short circuit or faulty wiring to the blend door actuator could prevent it from receiving the appropriate signal to close properly.
* Actuator itself: The actuator motor might be faulty, failing to fully close the blend door to the hot air side. It could also be getting stuck in an open position due to wear.
Troubleshooting Steps:
1. Check the temperature controls: Ensure the temperature dial isn't stuck or malfunctioning. If it's set correctly and only the side vents are blowing hot air, the problem is most likely mechanical.
2. Listen for the blend door actuator: When you change the temperature setting, you might hear a small motor clicking or whirring. If you don't hear it working near the side vent area, it's a strong indication of a faulty actuator. This usually requires accessing the actuator located in the HVAC housing.
3. Check fuses and relays: Although less likely, a blown fuse or relay for the HVAC system could be a contributing factor.
4. Inspect vacuum lines (if applicable): If your Grand Prix uses a vacuum system for blend door control, inspect the lines for leaks or damage.
In short: The most probable culprit is a faulty blend door actuator for the side vents. A qualified mechanic can diagnose and replace the faulty part. Replacing the actuator usually requires removing parts of the dashboard.
