I have had a rash of 2012's all with around 23 000 kms coming in idle only mode with code p207f and some times with p204f p20ee. Most of them have already had nox sensors installed and have now come back with the same code(s). I have had a couple with tail pipes that are dirty and have what looks like small bits of carbon on the inside diameter of the tail pipe. I have performed pinpoint test rd,ka and w and have not found anything wrong, inspected the down pipe for liquids, and have not found a bad egr cooler. I have ended up replacing the scr system in all of them but I don't understand why so many are failing like this. Just would like to know what you guys have been seeing and doing, it just feels like I might be missing something.