This sounds like a classic temperature blend door actuator that is not working.
Here is a guide to help us see what you are in for when changing the actuator out:
This guide may help as well:
Here is how to change the part on your car (below).
Check out the diagrams (below).
Let us know what happens and please upload pictures or videos of the problem.
Images (Click to enlarge)
Thursday, January 4th, 2018 AT 2:53 PM