What causes the AC to only blow cold out the defrost vents blows out the floor vents on a 1997 Buick park ave ultra?

If the AC only blows cold air out of the defrost vents and not from the floor vents in a 1997 Buick Park Avenue Ultra, it may be due to a malfunctioning mode door actuator or a vacuum leak. The mode door actuator controls the airflow direction, and if it fails or is stuck, it can prevent air from reaching the floor vents. Additionally, a vacuum leak in the system can disrupt the operation of these actuators, leading to improper airflow. Checking the actuator and the vacuum lines would be a good starting point for diagnosing the issue.