I had weak cooling a/c operation and took it into the dealer who advised there is a service bulletin to "adjust the 'manifold' door". I'm guessing to decrease the amount of hot air coming from the engine for heating the interior.
I signed up on the gm.com site as an owner but it really doesn't give any info I have found useful. Is there a site that we can find out about service bulletins?
The NHTSA website allows you to search for consumer complaints, recalls and service bulletins. It won't give you the full detail of the TSB but it is a good place to start.