Dash board air conditioning vents not working. Defrost and vents at feet are working on 2003 Chevy Silverado, 1500 series. What could be the problem.

Dealership has to repalce control unit and clear the trouble codes, a free fix is unpluging your hvac fuses, and disconnecting your battery for 3 minutes, and then reconnecting your battery, and pluging in your hvac fuse, ensure the controls are off so that the hvac can go through all its cycles to regain the correct positon. sometimes the control doors move out of indexed range and the computer doesnt know what position they are in, so they default to defrost. if yours isnt too far out of range you can go back to enjoying normal ac and heat, if it is too far out of range you can still have ac and heat just through the defrost postion.