Cannot switch from dash vents to defrost

Hello,

It sounds like you might have a faulty mode door actuator. The mode actuator is what changes the air flow from vent to defrost to floor.
It is likely stuck in the "on" position.
The mode door actuator is located under the dash on the driver the side footwell. Up towards the center console.

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.

The new actuator needs to be initialized properly; without it, it will not work correctly.
The ignition key must be off when replacing the actuator. Energizing the actuator before installation will cause the part to become positioned incorrectly and proper orientation will be lost. Incorrect actuator operation will result.

Another possibility is that the actual door might be jammed or has a broken gear, which does not allow it to turn even with the actuator operating correctly.
The only sure way to know would be to take apart the HVAC box and inspect/replace as necessary.

You will need to do some pinpoint checks to be sure the voltage is getting to the motor.

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-use-a-voltmeter

I attached the diagram for you to view. Check the connector for power with the key on.

Roy