Found the same TSB and an earlier one form 2018 GMC TSB PIT5643 . Found your response and a few others as well with the P1682 issue I've been having on my 2014 Acadia SLT. Notice quite a few fuses ( PCM ignition and ECM in particular) had blackened prongs/contacts on them, but not blown. I'm thinking that's indication of getting too hot. I went ahead and ordered the fuse box itself, it'll take 6-10 days and hopefully issue solved. We are lucky my wife works from home currently ( thank you covid?) so we really only use the Acadia for a few miles each day to drop the kids at daycare and then errand running on the weekends ( car is rarely ever running for longer than 30-45 minutes straight). Sometimes we go a week without issues, other times it'll happen a couple times in a single day. Very random.My wife’s 2014 Acadia recently had the P1682. After some research I found a tech bulletin about this issue: Bulletin No. 19-NA-276
I dug into it and I believe the problem is solved. It was the fuse block connection under the hood. The connection between the prongs of the upper section that mates with the terminal block female connection of terminal 51. Terminal 51 in X50A fuse block -underhood X3. The male prong/stab had a burnt spot/ electrical arc spot visible on it which meant there was not a tight fit in the male to female connection. I was able to increase the tension of the female connection by means of bending the female connection slightly. I also cleaned off the prong/stab of the male connection and reassembled the fuse block.
It’s been 2 weeks so far and the problem has not yet returned. It was happening on a daily to every other day. Included in the tech bulletin is the instructions for the repair by replacing the terminated lead of terminal 51 with a new terminated lead. I didn’t replace it but I did order the part and if it happens again I will replace it. Here are some of the pictures I took of it.
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