I am sorry, but this job is not for the novice DIY home mechanic, or those who are willing to try, however, “wondering is were do i start”.
Also, if your mechanic has made it clear, “it’s beyond his professionalism”, I would suggest that you find someone that has experience with the Buick ACC system, otherwise you will be just paying for “changing parts” until the problem is resolved! Again, this is a very complicated system to understand and diagnose with its network of electrical sensors, actuators, programmer and control panel. And finally, if you or your mechanic, do not have a factory shop manual to go by, or experience with the system, it would be best to take it to a professional.
(Dealer or AC specialist)?