DPF (Diesel Particulate Filter) or FAP is a device that is fitted to the exhaust system of every modern diesel vehicle to reduce emissions and meet European emission standards. It does this by trapping Soot (Particulate Matter - PM) from the exhaust gasses as the gasses flows 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 form the exhaust on occasions.

On most vehicles the regeneration process starts when the engine temperature exceeds 75C and exhaust temp above 220C and usually lasts 15-20 minutes. When these requirements are not met the regeneration process doesn't take place and thus the DPF getting blocked. This usually happen when you do a lot of short journeys and local driving. When the DPF blocks the engine will automatically put the car in limp mode to protect the engine from harm, thus limiting the speed to about 30-45 mph.