We did take it to different repair shops in different states, and the Mercedes tech. support line was even involved, but no one could figure out what was wrong. We took it to a so called certified MB Sprinter dealer and he refused to have his mechanic take a look as Freightliner was trying to get it right. We even asked if maybe they could see if something was missed. He refused. I complained to WB, sent an email to my contact at WB, back in July, with my concerns. He sent the email to his MB contact and asked him/her to get in touch with us. His contact ignored our requests, several times. I finally got a person to call us from MB after complaining to the corporate offices in Germany and USA. It as then that MB tech. support suggested the Nox Sensors be replaced, since so many other parts had been already replaced.
As for the resonator, we had that and the EGR replaced several times before WB owned up that water was entering the engine due to poor design of the front end of the coach.