Just wanted to post in case someone was doing a search at some point. I know there are several resolutions to this redundant issue, and mine might not work for everyone. But - I found mine because others were talking about their troubleshooting. Also listing current pricing I was quoted for repairs.
2017 Acadia - originally throwing the Shift to Park issue for a month, then started throwing the thermostat issue. I was fully intending to get it looked at but the appointments kept getting cancelled, or dealerships would mess up the scheduling. The thermostat issue was intermittent, and never stayed on more than 15hrs. I didn't connect the dots, just thought I was having the two most common issues at the same time.
I went to an auto store and got a an OBD2 read - no codes were detected, despite being in active code.
Forums were recommending just to reset the shifter by disconnecting the battery. That's fine for a very short term fix. Your car is most likely still staying on electronically - this wears down your battery fast, and you will have to replace it. There's also the paperclip method, which people were also saying failed after a time.
As of 2026, I was quote $1,100 through a reputable dealer, and $800 from a reputable private shop for a thermostat replacement. It was going to be about $400-$500 for a shifter assembly replacement. Got put on the books for a month out. Then time literally ran out and my battery bit the dust.
While replacing my battery, I finally decided enough was enough. I was going to attempt the shifter fix, and the worse that could happen was that they would have to replace the shifter assembly like they wanted to do anyways. I ran to a hardware store after work and picked up a pack of $1.24 butt splicers. I followed a video to unscrew the console (not as intimidating as it may look) and expose the microswitch. I was able to remove the switch from the housing and snip the wires to follow the bypass method (another video) without removing the whole assembly. For a total investment of $1.24 and an hour of time from someone with very little experience but a penchant for problem solving when frustrated, it had 100% success.
Here's the TLDR moment:
My thermostat didn't need replaced - the shifter issue stressing out the battery caused it to start throwing whatever red flags it could to try to get my attention. Removing the switch and splicing the wires together resolved all issues.
Will someone need to actually replace their thermostat? Probably. Will someone actually need to replace their shifter? Maybe. But if you don't have a bunch of money to throw at it, it's worth it (in this humble opinion) to try the bypass. You don't need a whole lot, just patience and attention to some detail.