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It's probably not even the thermostat. A bunch of people with the 2017 Acadia Denali have experienced that same exact symptoms as have I. It seems that if you unplug the Negative Battery Terminal that it clears these errors and everything starts working normally again. It worked for me.

I almost dropped $1400 to have the thermostat replaced. Then I did the battery terminal thing and cleared the errors. If it was the thermostat like I thought then the issue would have came right back as I have been driving it all over town and it's been in the 90s here in Arizona already. Been a week now and everything is working fine.

I think this is just another software gremlin with the 2017 Denali.

Interesting, I literally just got our 2019 back from the dealership (Friday) for having them replace the sensor. Thought it was good to go until I started driving home this afternoon and got the steering is reduced and realized the gauge isnt moving again. I am going to have my husband try your battery idea and see if it works. Not sure I want to go pay to have the thermostat done - when the gauge works its fine, hence why I thought it was just a sensor issue.
 

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Mike, I think you just saved me a whole weekend (full of cursing) and $150 parts to replace the thermostat and temp sensor myself, or $700 at a shop, or $1300 at my local GMC dealer. My '17 Denali had all the symptoms-- steering assist error, no coolant temp, fan running full blast, and no AC. I was about to order the parts, had already watched a couple YouTube vidoes to figure out if I could do it myself or not. Saw your post and thought, what the hell, I'll give it a shot.

So, I go to loosen the negative battery terminal... but it's already loose. Dealer replaced the battery three months ago, and apparently didn't adequately tighten the terminal nut. BUT! It didn't work right away-- it still gave me the error, no AC, etc., and I was still bummed. After about two cycles of 10-15 miles driving, engine off, start again, BOOM, the software must have finally reset. Temp gauge working, AC blasting cold, no steering assist error.

Anyway, thank you, Mike!!!

TL;DR: CHECK YOUR BATTERY CONNECTIONS FIRST! THESE SYMPTOMS CAN BE PRODUCED BY A LOOSE TERMINAL CONNECTION!
Update: :( Unfortunately, the thermostat symptoms are back... but intermittently. Battery connection is (now) tight. So, might be the sensor or the thermostat after all. Local non-dealer, chain tire and auto center quoted $1166 for thermostat, sensor, and coolant flush. Woof.
 
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