Fault code when going downhill

[Skman]

I have a 2016 Pete ISX15 2350 that I just bought and started working yesterday.
For two days in a row on the same trip...this code popped up with the amber check engine. Also the red light on the DEF gauge comes on and then goes out.
This happened twice yesterday and twice today at EXACTLY the same place. I was coming down a decent sized hill and had my Jake on and it was holding me at highway speeds pretty good. Then this code popped up.
I went about 40 miles to another hill...going downhill with Jake on...pops up again.
Last night I cleared the inactive code and it's there again tonight.
Happens in the same spots...both days. I guess my truck only wants to drive on flat land or up hills lol.

Any ideas why it would do this?