1. Blend Door Actuator (Most Likely Culprit):
* What it does: This small motor controls the blend door that mixes hot and cold air. If the actuator for the passenger side is malfunctioning or broken, it can't properly position the blend door to allow cool air to flow. It might be stuck in the "hot" position.
* How to diagnose: You'll likely need to access the actuator (it's usually located behind the dashboard). This requires some disassembly. Visually inspect the actuator for any obvious damage. You might try gently moving the blend door itself (with the system off!) to see if it's mechanically seized.
* How to fix: Replacement is usually necessary. You can find replacement actuators online (make sure to get the correct one for your model year). Replacing it requires some mechanical aptitude and patience. You'll need to consult a repair manual or online resources with detailed instructions for your specific vehicle.
2. Blend Door Cable (Less Likely, but Possible):
* What it does: Some older systems use cables instead of actuators. If you have cables, one could be broken, disconnected, or jammed.
* How to diagnose: Requires accessing the mechanism behind the dashboard. Look for any visible damage to the cable or its connections.
* How to fix: Repair or replacement of the cable is usually needed. Again, a repair manual will be helpful.
3. Vacuum Lines (Less Likely):
* What it does: Some climate control systems rely on vacuum lines to control the blend doors. A leak or blockage in these lines could prevent proper operation.
* How to diagnose: Inspect the vacuum lines for cracks, leaks, or disconnections. A vacuum gauge can help check for proper vacuum.
* How to fix: Repair or replace any damaged vacuum lines.
4. Control Module/Climate Control Head Unit (Least Likely):
* What it does: This is the brain of the climate control system. A faulty module could send incorrect signals to the actuators.
* How to diagnose: This is the hardest to diagnose. You might need a scan tool capable of reading climate control system codes. Replacing the control module is a more involved repair.
* How to fix: Replacement is typically the only option.
Before you start:
* Consult a Repair Manual: Get a repair manual specific to your 1999 Buick Park Avenue Ultra. This will provide diagrams, instructions, and torque specifications, making the repair much easier and less likely to cause damage. Haynes and Chilton manuals are good options.
* Safety First: Disconnect the battery negative terminal before starting any work on the dashboard.
* Consider Professional Help: If you're not comfortable working on car interiors, it's best to take it to a qualified mechanic specializing in older vehicles. The dash disassembly can be complex.
The blend door actuator is the most probable cause. Start your investigation there. Remember to take your time, be methodical, and refer frequently to your repair manual.
