Heater only blowing air out the defrost vents

Hello,

It sounds like you might have a faulty mode door actuator that is stuck on the defrost position.
The mode door actuator controls which vent the air will flow from. Sometimes they malfunction and get stuck in one of the settings.

Here is a helpful guide:

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

The mode door actuator is located behind the center of the instrument panel (see diagram below).

I have attached the trouble shooting flow chart below for your reference.

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

A very common cause of this is a vacuum leak or line off on the intake manifold or to the vacuum canister. The default position for the GM (and most others) that used the vacuum driven systems was to fail in defrost only if they lost vacuum, that way the windows could be cleared, and you would still get some heat. The first thing I would do is look at the black vacuum line that comes out of the firewall on the passenger side and up to the intake.