In Summary
  • DPF failure is usually caused by short drives with a lot of start stop traffic.
  • This prevents passive or active regeneration.
  • Failure to take the car for a long drive when the warning light occurs.
  • Using the wrong engine oil without low ash additives or fueling with poor quality adulterated fuel.
  • Frequent driving with low fuel level can prevent the active regeneration of the DPF.

Hello Paul, my BMW X5 2008 has of late started cutting power and smoking a lot through the exhaust. A mechanic, who checked it with a diagnostic computer says the DPF is faulty and I need a new exhaust. What is a DPF and what has caused its failure. Can it be repaired? Allan Kamugisha.

Hello Allan, diesel cars produce soot (particulate) when they burn fuel. This causes serious health and breathing problems. The growing need to curb the amount of exhaust fumes polluting the environment saw the introduction of Euro5 emission standard and legislation. This led to the design of Diesel Particulate Filters (DPF) fitted in your car exhaust to trap about 80 per cent of this soot. The DPF stores this soot in a particulate trap where it is regularly burnt to keep working properly.


This is called regeneration. During regeneration, excess soot in the particulate trap is burnt. This prevents harmful exhaust emission and black smoke. Your BMW DPF will regenerate passively by using exhaust heat during driving at speeds exceeding 60 KPH. Active regeneration occurs when the car will automatically deliver more fuel to the engine in order to increase exhaust gas heat and burn the soot.
The DPF signs of failure include an emission fault warning light on your dash or poor engine performance or loss of power as the engine computor elects a fail safe or limp home mode to prevent further damage.


DPF failure is usually caused by short drives with a lot of start stop traffic. This prevents passive or active regeneration. Failure to take the car for a long drive when the warning light occurs. Using the wrong engine oil without low ash additives or fueling with poor quality adulterated fuel. Frequent driving with low fuel level can prevent the active regeneration of the DPF.
The DPF can be serviced with special DPF cleaners and forcefully regenerated using software on a diagnostic computor. This depends on extent of DPF damage.