How to fix the "Speed Limited to 55 MPH" message when my DEF tank is full?

[BarnFindHank]

I’m in the middle of a road trip and my 2014 just told me I’ll be limited to 55 MPH in 50 miles because of a DEF issue, but the tank is full! What is causing this false reading? How do I reset the computer so I don't get stranded in limp mode?

 

[SootShooter]

It’s usually a bad DEF quality sensor or a glitch in the tank level float. What you can try is a "hard reset" by disconnecting the batteries for 30 minutes, but how you really solve it is by having a dealer or a shop perform a "Reductant System Test" to clear the internal counters. These LMLs are very sensitive to DEF quality.