Diesel Particulate Filter (DPF)

A DPF is a ceramic filter with thousands of channels closed alternately at each end, forcing exhaust gas through the porous walls while soot stays behind. It removes the vast majority of particulates by mass. The filter fills up and has to be emptied by burning the soot. That happens passively on long, fast drives, or actively when the control unit raises exhaust temperature. Ash from engine oil cannot be burnt and slowly builds up over the filter’s life.

The filter also collects ash from oil additives, which cannot be burned off. Over a very long service life this ash slowly fills the filter, which then needs professional cleaning or replacement.

A differential pressure sensor measures the pressure drop across the filter; the control unit also models the soot load. When it is high, the control unit starts a regeneration. Most filters have a honeycomb structure in which alternate channels are closed, forcing the exhaust gas through porous walls that hold back the soot particles.

Frequent short trips interrupt regenerations, and the filter can block. A DPF warning light, limp mode and codes such as P2002 or P2463 follow. Using the specified low-ash oil helps the filter last. Removing or disabling a particulate filter is illegal in many countries and causes a car to fail inspection. Low-ash engine oil to the correct specification helps the filter last longer.