Here's a breakdown of why and what to do:
* Blend Door Actuator: This is a small motor that controls the position of the blend door. It can fail mechanically (motor doesn't turn) or electronically (doesn't receive the correct signal). This is the most likely culprit.
* Blend Door: The door itself could be broken, jammed, or stuck in a position that prevents proper air mixing. This is less common than a failing actuator but possible.
* Vacuum Lines (Less Likely): Some older systems use vacuum lines to control the blend door. A leak or blockage in these lines could also cause this problem, but it's less likely given the consistent hot/cold split.
What to do:
1. Check the simplest things first: Make sure your climate controls are set correctly for a balanced temperature. Sometimes a simple adjustment can fix a seemingly complex issue.
2. Search for your car's blend door actuator location: A quick online search for "1999 Pontiac Grand Prix blend door actuator location" will yield diagrams and videos. Knowing where it is will make the next steps easier.
3. Inspect the Actuator: Once located, visually inspect the actuator. Look for any obvious damage or signs of failure. You might be able to hear it try to move by cycling the temperature controls.
4. Possible Solutions:
* Replace the Actuator: This is the most likely solution and relatively inexpensive. Many auto parts stores have them, and replacing it often involves only some simple tools and a bit of patience. There are countless videos online showing the replacement process.
* Lubricate the Blend Door (If Accessible): If you can access the blend door mechanism, a small amount of lubricant (like white lithium grease) might free up a stuck door. This is a long shot and only try this if you're comfortable with working on your car's HVAC system. *Don't over-lubricate*.
* Professional Help: If you're not comfortable working on your car's HVAC system, take it to a qualified mechanic.
Important Note: Before doing any work, remember that the refrigerant in your AC system is under pressure. Incorrectly handling the system can lead to injury. If you're not comfortable working with AC systems, leave it to a professional. The problem you describe doesn't involve the refrigerant itself, so it's unlikely you'll need to handle the freon.
