It could be a few things, but if I had to bet, it would be that you have a large chunk of crystalized def in the decomp tube and a cracked doser mounting flange. Theres a flex pipe between your DPF and SCR that has the tendency to allow DEF to dry up and restrict the ability of it to make it to the SCR. The computer has no way of knowing this, so it assumes that you have poor DEF quality. If you take that flex pipe out and clean it, it will fix the problem temporarily. There's usually a crack in the def doser mounting flange where it is welded to the bottom pan of the DPF also. There may also be upstream issues causing excessive DEF dosing, which can make the issue worse. Prime suspects are plugged EGR cooler and delta P ports.
there is a lot of things it could be usually there has to be some codes associated with this if not its a very very very expensive guessing game about what the problem really is you can start with nox sensor all the way to different engine components.