as far as doing this in a different thread, I was just trying to help. im not a pro on how they work, or what to do with them, but I do know enough to be dangerous with a wrench and an impact lol
DEF does go bad over time, its a pretty good shelf life so I wouldn't worry about it going bad in the tank, but if its at a low volume truck stop, I would hesitate to pick up a dusty bottle of it.
I've never purchased bad DEF so its not from experience, however, I would definitely use it as a strategy to avoid future problems.
if it was a vacuum leak, it could be all sorts of different problems that caused it to read bad. it could be bad or it just could have misread. I wouldn't just completely wright off the possibility of the DEF being bad, although it wasn't the source of this particular issue.
if you had a fuel mileage issue during this time or the up until this point, it could also be blamed on this issue. if there was a vacuum leak, it would be hard to get the optimal temp throughout the exhaust which would make it hard to do a proper regen.