Just posting a followup on my situation - post #168
While I could go on and on about how terrible of an experience I had with GMC and their customer service over the past 6 months, I'm not sure that everyone wants to hear the full details. In the end, after being
lied to multiple times by GMC about them covering the repair under "good will", then by the SCA warranty strangely, and then later by a refund check, I ended up covering the cost of the repair myself. I just want to point out that I never expected GMC to pay for the repair, they offered to help when I first contacted them, I took their fluffy bait, then got ditched.
I guess for those that are interested and to detail it some, keep reading, others, you can just ignore the rest of this post. In my case GMC offered a "good will" coverage and while the part was on order closed the account so the dealership could not get paid. We went through 4 different reps over the past 6 months and apparently they had no documentation on the agreed upon "good will" because the rep who got the approval and issued it was no longer employed at GMC (as were the previou 2 other reps I spoke with). It was just my word and the dealership's word - but apparently that wasn't enough. We provided notes (I have all the dealership notes as well), the dealership called multiple times, I spoke multiple times, but they would not accept them as proof.
On a followup call - GMC said it would be covered under their warranty (which quite surprised me) and I wouldn't have to pay for it, only two days later came back and said it wouldn't be covered because it fell outside the VIN check. Go figure. GMC apparently got in a heated conversation with the dealership - where GMC wanted the dealership to admit fault that they were the ones who said they would cover it and to then cover the repair so that GMC wouldn't look bad in front of me. I guess the shop supervisor at the dealership hung up on the GMC rep after a few choice words. I heard from both sides - very unprofessional the finger pointing that went on between the dealership and GMC. Later GMC admitted that they dropped the ball on the first "good will" issue and offered to refund the amount because of their mistake. They requested all my information to issue a check and said it would be sent in the mail.
A month later, the rep came back and said repair was once again declined and there was no longer anything that could be done. End of support. A simple no at the beginning of all this would have been a heck of a lot better than a - "get it diagnosed, we'll see what we can do, we'll pay for it, no we won't, we'll pay for it, no we won't, we'll pay for it, no we wont" over the past 6 months. It became a matter of principal after being lied to the second time... Over 24 phone calls, over 6 months of back and forth, it sounds like a bad novel, I just can't make this stuff up...
The repair was only for the driver side sensor and the harness. I am a little concerned that the passenger side sensor will fail this winter and I'll have to repair the sensor and another wire harness. This will be the 4th driver side sensor going in the 2015 Acadia, and the third wiring harness. So, my odds that the new sensor will also fail are pretty good.
I never expected to have the repairs covered, all I wanted was an explanation on why the vehicle I had, that exhibited the same issues multiple times fell outside their SCA. I wanted them to document the issue so that they were aware there were vehicles outside their scoped SCA that were affected.
They have no idea there are vehicles that have issues like this unless they are brought to their attention. I urge everyone who has this issue to file an NHTSA report and also contact GMC about the issue. Only with these additional reported issues will they ever expand the SCA or fix the GMLAN issue.
I'm still very concerned that the failing sensors pose a huge fire and safety hazard. The fact they draw power while shorting out is a huge fire hazard.
I'm also very concerned that the GMLAN offers no electrical interference protection - and that the sensor when shorted out can cause corruption which shuts down the airbags, and results in loss of vehicle maneuverability. These sensors malfunction due to water corrosion - which shorts them out, which corrupts the GMLAN. Many different things can cause them to short out, one fore example is a rear impact. Same basic principal - it's all just physics. Only a death will really trigger this to be addressed unfortunately. All I can say is don't get rear ended into oncoming traffic or your passenger airbag won't deploy.
In the end, I'm done with GMC. I won't own another vehicle, nor will I ever suggest it to anyone. I just rightfully can't recommend them. I've got it in the shop now to repair the rear evaporator core - because it's leaking freon. One more $1,000 repair. This vehicle is less than 5 years old

Due to the vehicle shortage I'm probably stuck with it for a while, so it's just a waiting game to foot the bill on another issue that crops up with it.
Oh, and one more issue I had to deal with. While the dealer had my vehicle they wrecked the front end and had to have it repaired. They attempted to overcharge me for a part that mysteriously was the same cost of the body shop repair. That part was for a 2018 Elantra... I guess they didn't expect me to look it up. They also tried to charge me for both sensors when they only repaired one. I had to make them crawl under the Acadia so that they could see that they did not replace the passenger side sensor. I'm done with that dealership as well.
I rarely complain, I rarely find things I dislike - I don't often do this, but I felt the need to post this update.
Just a terrible overall first impression with GMC from a new customer.