Modern diesel engines are quite clean, provided the soot filter is present and working properly. A soot filter can get clogged, or even malfunction. In that case the filter has to be cleaned or replaced. Unfortunately, sometimes the vehicle owner has the filter removed altogether. When the filter has been physically removed, it is ‘programmed out’ of the vehicle software, so that the removal of the filter remains undetected during an (E)OBD check. This is why in ever more countries a particle count is mandatory during a periodic inspection of e diesel vehicle.