A DPF (Diesel Particulate Filter), also known as FAP on some French vehicles, is a 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.

As with any type of filter, a Particulate Filter needs to be cleaned regularly to function properly. With DPF’s this is done by a process known as regeneration which involves a combination of a catalyst function in the 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 from the exhaust on occasions.


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 cause a fire risk. To prevent this damage, most new vehicles will go into “limp home” safety mode at this stage, meaning the vehicle will have minimum power, barely sufficient to crawl home.

For regeneration to take place, it requires the vehicle to be driven regularly at high speed on open roads e.g. motorway driving, typically driving at around 2500 RPM for around 15-30mins. This should be done at least once a month, as this will keep things working smoothly.

Many diesel vehicles are used primarily in urban areas, or on short, stop/start journeys, like local deliveries, taxis and school runs. Many of these have auto gearbox, so the vehicle does not get the chance to heat up sufficiently and is unable to carry out a full regeneration process.

Even if the regeneration functions as it should, over time the ash residue will build up and have the same effect, ultimately resulting in a blocked DPF. For this reason, a DPF is classified as a serviceable item by the vehicle manufacturers, like brake pads or air filters, and is not covered under warranty. DPF systems therefore need to be maintained properly.

European Mechanical Specialists