How does DPF regeneration work?
Regeneration is the process that burns trapped soot out of a diesel particulate filter, using heat made by the engine itself.
Regeneration is how a diesel particulate filter cleans itself. Soot collected in the filter is burned off at high temperature, turning it into a small amount of ash, so the filter does not clog. It happens automatically, but it only works if the car is driven in a way that allows it.
Understanding the process explains why diesel cars used mainly for short trips so often have filter problems, and why a warning light is worth acting on.
What actually happens during a DPF regeneration?
The engine control unit monitors how loaded the filter is, using pressure sensors before and after it. When the soot level reaches a set point, the car raises the exhaust temperature, usually by adjusting fuel injection late in the combustion cycle.
The soot then oxidises inside the filter. You may not notice anything, though some drivers see a slightly raised idle speed, a cooling fan running after switching off, or a faint smell. Check your handbook for what your car does.
What is the difference between passive and active regeneration?
Passive regeneration happens on its own when the exhaust is hot enough during a steady run, for example on a faster road at a constant speed. Active regeneration is started by the engine management when the soot load rises and passive cleaning has not been enough.
A third type, sometimes called forced regeneration, is carried out by a garage with diagnostic equipment when the car cannot complete the cycle itself.
- Passive: happens naturally on longer, steady drives
- Active: started by the engine control unit
- Forced: started by a garage using diagnostic equipment
Why does my car fail to regenerate?
The cycle needs the engine fully warm and the journey long enough to finish. If you keep switching off part-way through, the filter never fully cleans and soot slowly accumulates. Cold weather, stop-start traffic and short trips make this worse.
Faults can also stop it, such as a pressure sensor, temperature sensor, EGR valve or injector problem. Low fuel or old, contaminated oil can also interfere, because the cycle can add fuel to the oil in some engines.
Can I help my DPF regenerate safely?
Where the handbook says so, take the car on a longer journey at a steady speed, on a suitable road and within the speed limit, every so often. Do not switch off if you notice signs that a cycle is in progress, such as a fan running or a raised idle, if it is practical to carry on.
Use the correct grade of oil for a DPF-equipped engine and keep to the service schedule. These small habits do more for filter life than any additive.
When does regeneration need a garage?
If the warning light stays on, the car loses power, the fan runs constantly or the oil level rises, there is a fault that driving will not cure. We would read the fault codes first and check the sensors and related parts before anything else.
Depending on the vehicle, the next step may be a controlled cleaning, a repair elsewhere in the system, or replacing the filter. We can only advise after inspection.
Regeneration only works when the car gets hot enough for long enough, so your driving pattern matters. If your filter is struggling, Automaster near Totnes can look at it, so ask for a quote.
Common questions
How long does a regeneration take?
It depends on the vehicle and the conditions, so check your handbook. It is usually a matter of minutes on the move, and it goes more smoothly when you are not interrupted.
Can regeneration damage the engine?
Done as designed, no. Problems arise when the cycle is repeatedly interrupted or when extra fuel reaches the oil and is not dealt with. That is one reason to follow the oil change advice for your car.
Do all diesel cars have a DPF?
Many modern diesels do, but not every one. Your handbook or a garage can tell you whether your car has a filter fitted.
Can you clean a blocked DPF?
Sometimes a blocked filter can be cleaned or regenerated, but it depends on how badly it is loaded and whether it is damaged. We would need to inspect it before saying whether cleaning is likely to work.
More advice
Ask for a quote
Tell us the make, model, registration and what the car is doing, and we will come back to you. Prefer to talk? Call our quote line on 01803 862152.
Automaster, Longcombe Cross, Longcombe, Totnes TQ9 6PL, United Kingdom