Most likely your mode door actuator has failed. Here is the procedure for replacing it.
What happens is when this actuator fails it puts the door in defrost mode as a default because it is a safety issue is your defrost does not work. If it doesn't then your windows will fog and you will have no way of clearing them. So anytime it blows on just defrost, it is most likely the mode door actuator. There is a chance that it is wiring or the control module but this is rare and unlikely.
If you would like to test the actuator, I included the ohm readings that you will need to find in order to have a functional actuator.
To test this, simply put your multimeter on ohms and measure each pair of pins.
Images (Click to enlarge)
Thursday, May 9th, 2019 AT 7:17 PM