2017 Thor Motor Coach Compass

While driving at highway speeds (70 mph+/-), my diesel powered vehicle 'derated' (I believe that is the term) for no apparent reason. An engine light came on related to a DEF system malfunction. I can provide the code later. My vehicle instantly went from 70 mph to 15 mph on a crowded interstate, on an inside lane. With some difficulty I managed to make across three lanes to the shoulder and turned off the vehicle. After a quick inspection, finding nothing obvious, I proceeded down the shoulder to a gas station. I tried to top off my DEF tank, but it was full. After an hour the engine light went off and the vehicle drove normal. So I'm back on the highway. I went another hour, the engine light came on and the vehicle derated at highway speed. Same white knuckle lane crossing. This time I went to a truck repair shop, who diagnosed it as an issue with the DEF system. They reset the engine light and the vehicle seem to drive normally again. This time we management to make it home without an additional incident. I went to a local diesel mechanic (Ford refused to honor the warranty) who replaced the DEF pump assembly. I think it was specifically the float arm, or the mechanism that measure DEF tank levels. It cost over $1000. While I have several issues with the situation, the primary one is that a perfectly sound vehicle purposely derating the speed from 70 mph to 15 mph in heavy traffic on an exceptionally hot day. Going across three lanes of interstate traffic decelerating to 15 mph, was more than a little scary. Someone could have easily been killed. And the fact that this system is designed to shut you down over exhaust emissions is insane and dangerous. The easy analogy would be derating a gas vehicle over an oxygen sensor failure. I'm hoping that if enough other people complain about this, the government will fix a problem they created. The craziest thing about this is my DEF pump was actually working.