This is normally the mode door actuator. When they fail they default to the defrost position. This is so that no matter what the windshield will not fog or ice over. It becomes a safety issue. I included the process on how to replace this.
Here is a guide that talk in more detail about this issue:
Let me know if you have questions.
Images (Click to enlarge)
Wednesday, March 25th, 2020 AT 3:40 PM