Derate and stop engine

Won't hurt anything but your schedule. LIkely a failed level sensor.

 

There are several things that can cause it to show low DEF or out of DEF that actually have nothing to do with the DEF level or the gauge working properly. It is the (flawed) logic the system uses, usually when NOX efficiency is low due to failed sensors, bad DEF pump or metering, etc. The only purpose of the DEF is NOX conversion, so the system will "assume" if NOX efficiency is low that there is no DEF being pumped into the SCR system. It will also often say DEF Quality Low and in both cases the quality is fine and there is plenty of DEF in the tank. But a shop is going to charge you as the check all of those systems, and mostly the emissions can only be worked on at a dealership so it's top dollar for this work. At first you will derate power around 25%, keep driving and it will derate speed to 55mph, and finally if you keep going it will derate you to 5mph. All this is not a decision of the manufactures it is EPA requirements for the emissions systems (Thank You CARB).

 

roshea is spot on.....