My 2016 5.3L Sierra (SLE) 4x4 has had been experiencing the issues in the thread title. $2,800 miles on her, two months since delivery as of 3/26/16. Temperature gauge will not rise, radiator fans stuck in high mode, and AFM will not enable V4 mode. I take the same route to work daily and know all the stops V4 runs. I have one 10 mile stretch of totally flat highway.
It happened three times this past week, each time was on the initial drive of the day. During stop and go traffic the second time I shut off and restarted the truck which didn't do anything. When I arrived to work shutting of the truck, waiting 5 minutes and the restarting had no effect. Dealer was unable to pull a code or reproduce last week. No check engine light or other indicators.
Yesterday I was running errands and it occurred and lucky for me I was visiting my parents and my home town GMC dealer is open 10-3 so I wung it in and left it running. Service guy comes out and says 'Hello, what seems to be going on aside from this thing making a ton of noise'. I chuckled. I bet not many of you have heard the radiator fans stuck in 'Coolant Hot / overheat mode'. YOu can hear it in the cab, clear as day with the radio on. In the service area it was absolutely deafening. We had to step outside and shut the doors to talk to each other while his scan tool ran. The only comparison I had it to those car wash blowers that dry your car at the end, right when you are under them.
I did a search and manually browsed 10 pages of topics, didn't seen this one. I'll report back what the dealer finds if anything, and continued troubleshooting or resolution. In the event someone else has this issue they may find value in the information.