That same problem occured to a friend of mine on his '12 MB 188 hp. It took many part changes, including the two nox sensors, and both computers. He ended up talking to a MB mechanic that told him to "insist" on changing both computers. As I understood what was told to him, there is the main vehicle computer and the emmisions computer. The programs randomly conflict in communication, thus tripping the def warning, and that results in the, so many starts left sequence. They finally caved, replaced the both computers and wella, 20K later, no def issue.
