In this video, we replace the thermostat in a 2015 Jeep Grand Cherokee because of a code P0128. Pretty easy job as long as you don't break anything like I did.
Thermostat & housing: https://amzn.to/3fYDvZb
Mopar coolant: https://amzn.to/2SJJedG
Purge tube: https://amzn.to/3c24C4w
P.S. To be fair, there's actually a service bulletin on that purge hose. It is a known issue on this vehicle. I'll fix it for the owner (who is a friend & neighbor) when this repair fails.