Short answer

DPF regeneration is the process of burning trapped soot out of a diesel particulate filter. It happens passively at normal exhaust temperatures, actively when the engine raises exhaust temperature on purpose, or as a forced regeneration started by a technician with a diagnostic tool.

Dpf regeneration
Regeneration oxidises the carbon soot collected in a DPF into carbon dioxide, restoring the filter's capacity. It does not remove ash.
On this page

Why a DPF needs regenerating

A diesel particulate filter traps soot on its ceramic walls. As soot builds up, exhaust flow becomes restricted and back pressure rises. Regeneration burns the soot to restore flow. Without it, the filter would block within a short time.

The three types of regeneration

Passive regeneration

On many vehicles soot burns continuously when exhaust temperatures are high enough, helped by a catalyst that produces nitrogen dioxide, which oxidises soot at lower temperatures than oxygen alone. This happens without the driver noticing, mainly during motorway or sustained higher-speed driving.

Active regeneration

When the ECU estimates that the soot load is high, it raises exhaust temperature, usually with extra fuel injection late in the combustion cycle, to around 600 °C so the soot burns. Drivers may notice a higher idle speed, the cooling fan running, a hot smell or slightly higher fuel consumption. If the journey ends before regeneration finishes, it may restart on the next drive.

Forced (service) regeneration

A technician can start a regeneration with a diagnostic tool while the vehicle is stationary. It is used when active regeneration has failed, and should only be done after checking why. Forcing a regeneration on a heavily overloaded filter risks overheating and cracking the ceramic. More detail: passive, active and forced regeneration.

Regeneration vs cleaning

Regeneration and cleaning are often confused. Regeneration happens on the vehicle and only removes soot. DPF cleaning, particularly off-vehicle cleaning, removes soot and ash. If a filter keeps needing regenerations, fails to regenerate or has high mileage, cleaning may be the next step. See regeneration vs cleaning vs replacement.

When regeneration goes wrong

Frequent short journeys, faulty sensors, engine faults, a low fuel level on some vehicles or an empty additive tank can all prevent regeneration. If regeneration keeps failing, see DPF not regenerating.

What happens during an active regeneration

  1. Soot load estimated

    The ECU uses the differential pressure sensor and its own soot model to decide that the filter needs regenerating.

  2. Exhaust temperature raised

    Extra fuel is injected late in the cycle and burned in the oxidation catalyst, heating the DPF.

  3. Soot burns off

    At high temperature the trapped soot oxidises to carbon dioxide.

  4. Load checked again

    Pressure readings confirm the filter is clear and normal running resumes.

In this topic

DPF problem

DPF not regenerating: causes and fixes

Why a DPF will not regenerate or keeps failing to finish, what to check first and when a forced regeneration or cleaning is needed.