It sounds like you have temperature blend door actuator that is out and needs to be replaced. Here is a guide that has a video showing you how to fix the problem:
Make sure hot coolant is getting to the heater core by checking the heater hoses in the engine compartment. Make sure the blend doors and temperature controls in the heater box are functioning properly.
Here are the diagrams to change the temperature air door motor (below).
Please let us know what you find.
Images (Click to enlarge)
Thursday, December 7th, 2017 AT 8:30 PM