I actually have a 2017 Acadia Denali and mine started doing this exact same thing about 6 months ago, but a couple days later, everything went back to normal. I read on this forum that the Coolant Temp Sensor was the main problem on the older models, so I went to the GMC dealer and purchased a new sensor. I figured I would wait until it problems happen again, and then change the sensor. However, the Acadia ran great all summer, until last week, the temp gauge dropped to 0, the air blew warm, the rad fan was racing, and the "Steering Assist is Reduced " warning came on the dash. I drove the car the next day and in the middle of my drive, everything started working again. Then last weekend, I finally had time to replace the Coolant Temp Sensor. It is not in the same place as everyone says for the older models. It took me 3 hours to find it, but I finally did and replaced it. It is actually on the top of the engine, left side under the air intake cover. I thought everything would be fine, but then today, all my problems came back. Now I'm puzzled, what is wrong with these that this problem keeps coming back? Do you think its the thermostat?