Had that happen on few trucks, turned out the def injector was faulty, same problem on a few hundred trucks acording to the dealer...
Anything that goes wrong with the aftertreatment system will eventually shut your truck down, it varies per engine manufacture on the time. You need to get this fixed asap. If you do not it might shut your truck down in a spot where you may not like it. Trust me when I say this as I have been on the side of the road several times because of this.
I have had to haul 3 kenworths to the dealer so far for this same reason. Good thing they only derated or it would have been a pain to get them on the lowboy... LOL
Just went though the same thing with a DD15 in a 2016 Cascadia. 35k miles and a wire harness melted, as well as a plug into the DEF pump corroding, and the pins fusing into the pump. They had to replace the pump and go through the wire harness to make sure it was all sealed up. My DEF low level was flashing but showing full as well. The check engine light was on. The truck also derated 25% till the wire harness dried out.
I'm wondering is it just the fact that these engines and trucks are not ready for such strict emissions. I'm being honest when i ask that Question. I've read multiple reviews with other users on here. Seems like no matter what engine you have. You will eventually have issues with the DEF. No matter how new or used it is.
The Cascadia has the cheapest of the problems. Right now I can tell the Def pump and wire harnesses are the weakest points on this engine. Beats replacing EGR valves and coolers every 120k like you would on a MAXXFORCE.
I've had a Maxxforce engine. It was just as if not more sorry than the other three engines I've dealt with. Consistently blowing blue smoke check engine light would come on. It was nerve wrecking.