I had the same problem with my 2003 Trailblazer after replacing the battery. Dealer said that the vents needed to be reprogrammed. When that failed he replaced the actuwaiter motor ($496.00) that operated the vents and then reprogrammed the computer that operates them. Had noise problems after that and it's in the shop today, they say that GM tech folks have a new computer program that prevents it from trying to go from ac to heat and back all by itself (that was the noise). So in answer to your question your computer probably needs reprogramming.