2016 Silverdo LS … only blows the heat through dash vents.
The lights on the selector buttons light up indicating Defrost or Floor or a Combination … but no change in air flow.
The preferred method of diagnosis and repair would utilize a scan tool. I am going to assume you do not have access to one so I would recommend performing a recalibration of the actuators as a first step. The GM service information is a little vague on this subject, if you do not have a scan tool. I added some extra information. If performing the recalibration does not rectify the issue I would recommend replacing the mode actuator.
Alternate Method (without Scan Tool)
Do not operate any HVAC controls while the HVAC control module is calibrating as this may interrupt the process. If interrupted, improper HVAC performance will result.
Ignition OFF/ Vehicle OFF.
Remove the HVAC control module fuse for a minimum of 10 seconds. There are two fuses listed for this step F26DL and F34DL. They are both 10 amp and located in the left interior fuse box. (I would try performing the procedure first by using the F26DL fuse. If that is unsuccessful I would use both for the procedure.)
Install the HVAC control module fuse.
Start the vehicle.
Wait at least 40 seconds for the HVAC control module to self-calibrate. pay attention to see if the mode actuator moves during the recalibration.
Turn the ignition off for at least 10 seconds but no longer than 30 seconds.