Driving with a 4 horse trailer attached and loaded, truck reported speed would be limited as DEF fluid was low. I topped off the fluid, and it still limited speed. Every 60 miles or so it got slower and finally limited my speed to 4mph even though DEF was totally full. I crippled into a truck stop with the horses in tow. Fortunately, we had another vehicle 2 hours away that could come get the horses and trailer while I waited for a tow, just about 2 hours (normal speed) from home. The DEF was full, there was no reason to limit the speed. The tow company came and said they could reset the code. Unfortunately, the EPA required that this type of code only be reset by dealer. Had to have the truck towed. Turned out to be a faulty exhaust sensor that just needed to be reprogrammed. Come on, limiting the speed to 4mph just because of what the EPA believes is a carbon emissions issue with low DEF?? That is ridiculous!