* The blend door actuator may be faulty. The blend door actuator is responsible for opening and closing the blend door, which controls the flow of air from the vents, dash, and defrost ducts. If the actuator is faulty, it may not be able to close the defrost door properly, causing air to continue flowing out of the defrost vents.
* The blend door itself may be damaged. If the blend door is damaged, it may not be able to seal properly, allowing air to flow through the defrost vents even when the door is closed.
* The vacuum lines that control the blend door may be damaged. If the vacuum lines are damaged, the actuator may not be able to receive the vacuum pressure it needs to operate properly.
To diagnose the problem, you will need to remove the dash panel and inspect the blend door actuator and blend door. If either of these components is damaged, it will need to be replaced.
Here are some steps to help you remove the dash panel:
1. Disconnect the negative battery cable.
2. Remove the screws that hold the dash panel in place.
3. Carefully pry the dash panel off of the dashboard.
4. Inspect the blend door actuator and blend door.
5. If either of these components is damaged, replace it.
6. Reinstall the dash panel.
7. Reconnect the negative battery cable.
8. Test the heating-AC control to make sure it is working properly.
