What are the symptoms / code? My dealer topped off the DEF tank and and a couple of hundred miles later the instrument panel displayed a DEF message and to see dealer. I also got a Uconnect email. The stored code came up as a knock sensor. I questioned it and the service advisor said they were related.
Dealer changed the knock sensor and all was fine. Have the dealer check it. Unless the DEF was old and expired I doubt if it was the problem. I used Peak in my 2014 and never had a DEF message. The above incident was with my 2015.
I used the peak blue tec, never had an issue, I was feeling cheap and bought the Walmart brand and 15 miles later, it's saying i have the wrong stuff. I went ahead and found the tank, it's under the back, all the way in the back center, right in-front of the hitch.. It has a snap on hose, was easy to get off. I just emptied it. I'm going to go by the blue def this morning, and put in 2.5 gallons. Then if need be, I'll reset my computer.