What is a DPF?

A DPF (Diesel Particulate Filter) is a filtration device fitted to the exhaust system of modern diesel vehicles to reduce emissions and meet European emission standards. It does this by trapping Soot (Particulate Matter – PM) from the exhaust gasses while letting the gasses flow through the system and out through the exhaust. On certain French engine vehicles there is an additional fuel additive system that works alongside the DPF, known as a FAP system. Common vehicles with FAP are Citroen, Peugeot, Renault, some Fords and Volvos.

As with any type of filter a Particulate Filter needs to be cleaned or replaced regularly to function properly. With a DPF this is done by a process known as Regeneration which involves a combination of a Catalyst function in the DPF system and burning the soot to gas at a very high temperature leaving behind an Ash residue within the DPF. Regeneration should be an automatic process taking place in the normal use of your vehicle, you may have noticed this in the form of a blast of white smoke form the exhaust on occasions.


DPFs have been in common use in passenger cars and light commercials from around 2003 in preparation for Euro 4 regs (2005), with Peugeot, Renault and BMW being early takers. Euro 5 (2009) made it compulsory for diesel cars and light commercials to have a DPF fitted and Euro 6 (2014) has tightened this up further. Euro 6 also made it compulsory for HGV’s to have DPF’s fitted although many HGV’s have had DPF’s fitted long before then.

Euro 6 introduced the measurement of Particle Matter (PM10) in vehicle emmissions as well as drastically reduced permissable Nitrogen Oxide (NOx) limits. Vehicle manufacturers have used DPF, EGR and SCR (AdBlue) systems since Euro 5 to comply with the stricter regulations. From Euro 6 light diesel vehicles are also being fitted with SCR and it is expected that Gasoline Particle Filter (GPF) will be fitted to petrol veicles in the near future.

Problems encountered with DPF

If regeneration doesn’t function properly it leads to a build up of soot affecting performance and fuel economy, left unattended this will result in a blocked DPF which can ultimately cause very expensive damage to other engine components. A blocked DPF is also potentially dangerous as it can cause overheating in the exhaust system and in extreme cases cause a fire risk. To prevent this damage most newer vehicles will go into “Limp Home” safety mode at this stage, meaning the vehicle will have reduced power barely sufficient to get home.

truck cylider dpf

For regeneration to take place it requires the vehicle to be driven regularly at some speed on open roads e.g. motorway driving, typically driving at around 2500 RPM for approx 30mins at least once a month, this will keep things working. However, normal driving in a diesel vehicle does not maintain 2500 RPM constantly and many diesel vehicles are also primarily used in urban areas or on short journeys e.g. local deliveries, taxi’s, school runs etc so the vehicle does not get the chance to heat up sufficiently and is unable to carry out a full regeneration process. This is more so on vehicles with an auto gearbox or larger capacity engines i.e. over 2500cc as they rev lower than manual vehicles with smaller engines.

dpf cut away
Even if the regeneration functions as it should, over time the Ash Residue will build up in the DPF unit. When this happens the vehicle will not be able to regenerate fully and have the same affects above ultimately resulting in a blocked DPF. For this reason a DPF is classified as a Serviceable Item by the vehicle manufacturers in the same way as oil filters or air filters etc and not covered under warranty. The typical lifespan for a DPF unit on a new car is around 80,000-100,000 miles. DPF systems therefore need to be maintained properly. If you not clear on how to maintain your DPF please call us for advice.

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