Def light

Looks like a bad outlet NOx Sensor.
It will shut you down to 5 mph , probably today sometime.

 

I have the same problem ( def light flashing, engine and mil light) Happens when pulling heavy load up steep hill. At 20 miles it derates to 55 for exactly 30 miles then problem goes away. If lights are still flashing when I park it for the night, the next morning it’s derated to 5 mph. I’ve replaced filter, def pump, doser awning kit, both nox sensors and harness. Still same problem. If you hold clutch down just enough to activate sensor, you can get it up to 45 mph when in 5mph derate. But only if you are bobtail or empty trailer. If you are loaded, then disconnect or get tow truck.

 

There is an aftermarket device you plug in that allows you to do a forced regen and clear codes.

 

Yes, emission problems guarantee that you will be making mechanic rich while you jack up your credit card.