It's an ECM error. I got it twice after flashing when I had my PPE Hot +2. The answer I got from PPE was a communication error. They said to clear it and everything was fine.
-Kyle
Diesel engines do require some back pressure to work. But we don't have to worry about it as much since the turbo is creating the backpressure. If you Dyno your truck with the stock exhaust (cat and muff) then dyno with a straight pipe your low end torque will be slightly lower.
On the EGR...