2011 X35D - P20EE issue

[Josh95mx]

So I have had my X5 for about 7 months now as my 540 was totaled out, I have put about 5K on it since I got it, its at 84K now. Its been great, but we were on a long trip from Seattle to Portland and it threw a P20EE. Looked some things up and seemed to be an array of things that could cause this. I read a couple places that driving long duration in excessive speed could cause it to throw the code. then on the way home We got the improper exhaust fluid code and 200 miles to drive until shutdown.

the X5 spent all last week at the dealer to get it resolved. They changed the DEF out and would not go away. they ended up having to put it through a drive cycle and the everything reset. the very next day the P20EE came back. Now they are telling me its the Cat. I am kind of inclined to think its the NOX sensors or the DEF injector. any thoughts on this? as it was the Def change was a grand, and this is 2500...

Thanks for any input.

[SlackTube]

X5 diesels have Nox sensor issues, knock sensor issues, def injector issues, DPF issues, carbon buildup EGR problems, Def delivery module problems, both active and passive tank issues, air meter issues and yes cat issues.
a diag over the Internet is basically impossible but I'd be inclined to trust what the dealer says ( I know ).
good luck