My 2016 Sierra 2500HD has 176,000 miles on it already and recently started eating parts. The latest part to fail was the reductant tank heater (DEF Heater). Upon cold start the check engine light came on with code P21DD. After a little reading I learned that this can result in “Limp Mode”. I was in the middle of nowhere in Northern Wyoming. Not cool! I drove down to Denver and replaced it that week. I shot this video of the repair.....