Trouble as shifting would be done through pre-programmed valves. Not familiar with Buicks (not too far ahead of you with '16 model) or location of that particular valve. May be internal or perhaps can be changed without xmission removal?

But now along with a valve only change idea I'd go with a new filter and fluid. At least they used to have a pad type internal filter. Work on my own cars only once in awhile these days, nothing wears out as retired. Almost easier to buy a new car than tires for existing lol. Still rotate my tires as example so I am familiar with flat change, was a pleasure doing my LaCrosse.

Not attempting to second guess dealer but if complicated a new transmission (rebuilt) may be cheaper than labor expense tearing into existing. Yup, have repaired shift band trouble when those were used. Didn't have to remove entire transmission, only the pan. Things as such can be so far-fetched. https://en.m.wikipedia.org/wiki/Automatic_transmission http://freeasestudyguides.com/transm...rake-band.html

OK, maybe explains a lot: https://www.carid.com/2012-buick-lac...control-units/ Appears this may be a solinoid control module after looking closer but new transmission, maybe not.