DPF Regeneration

Each time diesel burns in the engine, soot is produced, and the filter traps it before it can exit the tailpipe. Bit by bit the filter fills. A regen pushes the temperature inside high enough for the soot to burn, leaving only a little ash behind, as Noregon explains.

How much heat is required varies with the filter. A catalyst coating lets soot burn at 250 to 350°C, but filters lacking that coating can need as much as 550°C. Lab testing covered by DieselNet found soot burned fast and fully only near 600°C.

Ash left over after burning will not burn away itself. It accumulates gradually, and given enough time it can harden and harm the filter. That is why a DPF generally lasts about 100,000 miles before replacement is needed.

It will — in two ways, and usually you will notice neither.

Passive regeneration takes place on its own during normal driving once the exhaust is hot enough to burn off soot. In long-haul trucks, this filter must climb above 1,000°F for that to happen. Short journeys and cold conditions often keep the exhaust from getting that hot, and vehicles that seldom cruise at highway speed for long periods rarely reach it either.

If passive regeneration falls behind, the vehicle steps in with active regeneration. The engine control unit, which is the computer running the engine, kicks it off once soot in the filter hits a set level. It sprays in extra fuel to lift the exhaust temperature — Snap-on puts the typical figure at 600 to 700°C, or about 1,100 to 1,300°F.

If the filter keeps clogging and the vehicle fails to burn the soot away by itself, what remains is a forced regeneration — one that a person sets off rather than the vehicle. A technician typically triggers it using a scan tool, a handheld device that connects to the vehicle's computer and can order the engine to start a regen.

A forced regen accomplishes the same thing as an active one: it burns the soot down to ash. The only difference is that a person requested it rather than the engine control unit deciding the moment had come.

Since ash remains in place, a forced regen will not restore the filter to new. It removes the soot, yet ash continues to collect until the filter finally needs replacing.

That orange light on the dash comes on once the filter starts to block up, as BookMyGarage notes. DPF warning lights may be solid or flashing, and each signals a different level of concern.

The light is important because the trouble worsens when nothing is done. When a regen that is needed never happens, the engine can derate — it deliberately cuts power to protect itself. The vehicle might fall into limp mode, running only at reduced power, or it might not be driveable at all. With enough blockage, some engines refuse to restart.