Buick Repair Questions? Ask a Mechanic for Answers ASAP
This would be due to a failed mode actuator. There is an electric motor (actuator) on the side of the heater case that moves the doors to direct airflow through the floor vents, dash vents, or defrost vents. Over time, the actuator wears and feedback goes outside of the normal limits. When this occurs, the actuator is forced into a default mode, usually defrost. To fix the problem, the mode actuator must be replaced.
The actuator is located behind the middle portion of the dash. If you go under dash on left side, remove plastic cover then look up past the accelerator pedal to the side of the heater case. The actuator with the large plastic cam attached is the one that is faulty.
You can check it on the car. Remove the HVAC fuse and then reinstall fuse. Turn the key on, do not touch any of the a/c controls. Actuator should cycle through all of the modes. If it does but then returns to defaullt mode, the actuator will need to be replaced.
The actuator is not too difficult to replace. Total time for replacement is usually under 1 hour.
The video in the link below shows the location of this actuator. Please note that the person that posted the video does not replace the actuator, they basically bypassed it to get airflow from the dash.
The Rainier and Trailblazer (as well as the GMC Envoy) are all built on the same chassis and share the same components behind the dash. Dash layout is a little different but the heater case and actuators are identical.