The particulate filter only burns off if the engine runs at operating temperature for long enough. On short, urban journeys the regeneration cannot even complete, so ash builds up in the filter, and the end of it is limp mode.

The regeneration needs a motorway run, and 20 to 30 minutes of it is the measure. A shorter journey starts the process but does not finish it, and an incomplete regeneration does damage of its own: the post-injection mixes diesel into the sump oil.

The life of the filter is also extended by a forced regeneration carried out with the diagnostic tester, done between 30 and 40 thousand kilometres.

A saturated filter means lack of power, limp mode and finally a filter replacement. An injector that atomises poorly leads to the same place, because it produces more soot.