Short answer
Passive regeneration burns soot continuously during normal higher-speed driving. Active regeneration is started by the engine computer, which raises exhaust temperature. Forced regeneration is started by a technician with a diagnostic tool while the vehicle is stationary, usually after active regeneration has failed.
On this page
All three types of regeneration do the same job: burn soot out of the diesel particulate filter. They differ in what starts them and when they are used.
Passive regeneration
During sustained driving, exhaust temperatures and a catalyst that produces nitrogen dioxide allow soot to burn steadily. Vehicles that do regular longer journeys may rarely need anything else.
Active regeneration
When the soot load estimate reaches a threshold, the ECU raises exhaust temperature, typically with extra late fuel injection, to burn the soot. If you switch off mid-regeneration it will try again later, but repeated interruptions let soot build up. See DPF not regenerating.
Forced regeneration
A technician uses a diagnostic tool to run the manufacturer’s regeneration routine with the vehicle stationary. It is a repair step, not routine maintenance, and the reason active regeneration failed should be found first. Costs are covered in DPF regeneration cost.
Passive vs active vs forced regeneration
| Aspect | Passive | Active | Forced |
|---|---|---|---|
| Started by | Normal exhaust heat, helped by a catalyst | The ECU, based on soot load | A technician with a diagnostic tool |
| Driver involvement | None | Needs a long enough drive to finish | Vehicle taken to a workshop |
| Typical conditions | Sustained higher-speed driving | Any driving once triggered | Vehicle stationary, engine running |
| Signs | None noticeable | Higher idle, fan running, hot smell | Done under supervision |
| Removes ash? | No | No | No |