If a unit it needs to go out with a loaded trailer ,if a rigid it has to be also loaded …Take it out ,get all the exhaust temps above 200 degrees ,when back at the workshop they have to if they have an old DAVIE XD go to regulation functions ,overview and read the NOX sensor PPM, if using the new DAVIE more likely go to monitoring and look at the NOX PPM and record it ,now using your controls set the revs to 1200rpm and just sit there for 5 mins ,there is a different parameter depending if a drum silencer or a stack …Now you are just sat there at a fast idle so you are checking the NOX sensors only so the reading in and out should be pretty consistant ie 200 ppm moving up and down slightly if any reading is erratic and all over the place that sensor is faulty and any reading which is I think above 220 ppm then again that sensor is faulty.He should be able to get the exact figure as there are bulletins