What year is yours? At the end this spring just before we sold our 2013, we found out two big causes of my issues.
Number one....because of all the other problems I had with it, the dealer mechanics poked holes in all the wire harnesses with a test light and the harness that controls the DEF system corroded.
Number two....apparently there are bungs inside the One-Box that are known for cracking and breaking off. And I think what was explained to me is the pipes inside go out of alignment? I was always wondering why mine when it was getting close to doing a parked regen would make trumpet sound in the exhaust. That was the cause. Had to replace the whole one box.
When my old DD15 ran right, I was happy with the power. With just the truck I can gross 80,000 and 140,000 with the pony. All our trucks have Eaton 18 double low, double overs with 4.30 rear gears on 22.5 rubber. (my new one has 4.10's)
And compared to our Coronado 122SD that had a 600/1850 ISX in it......from a standing start I could smoke our driver. It would accelerate and grab gears faster. But come a hill, I could hold equal until I had to drop that first gear. Then he would wave good by.
The way it worked out because I drove all three one day doing the same job, same loads...the ISX was 10 MPH slower on the hill than my 550 Cat and the DD15 was 15 MPH slower than the Cat. (on that particular short hill.)
And a hill that I could maintain 55 MPH with the ISX, the DD15 would drop to 40.