A diesel particulate filter (DPF) is a filter that captures and stores exhaust soot (some refer to them as soot traps) in order to reduce emissions from diesel cars. Euro 5 exhaust emissions legislation introduced in 2009 to help lower car CO2 emissions effectively made DPFs mandatory, and since then, around one in two new cars a year have been diesel-powered.

Short journeys at low speeds are the prime cause of blocked diesel particulate filters. This is why car makers often go as far as recommending city-bound short-hop drivers choose a petrol car instead of diesel (and it’s why diesels are something of a rarity in the city car sector). Other things that are bad for DPFs include poor servicing. A diesel particulate filter on a poorly serviced car may fail sooner than a well maintained one, generally, they should last for at least 160,000 Km. Performance modifications can damage a diesel particulate filter, as can using low-quality fuel and even running the car frequently on a low fuel level as the car may avoid DPF regeneration in order to save fuel.

If the DPF is becoming clogged with soot or a fault develops in the system, an orange light will typically appear on the dashboard. They usually look like a piped box that has dots in the middle, although they can slightly vary by manufacturer, check your handbook for more information.

Because diesel particular filters only have a finite capacity. It traps ash and soot periodically. Soot has to be emptied or ‘burned off’ to regenerate the DPF. This regeneration process cleanly burns off the excess soot deposited in the filter, reducing the harmful exhaust emission and helps to prevent the tell-tale black smoke you used to see from diesel vehicles, particularly when accelerating. This regeneration process leaves ash behind and is incombustible and must be removed from the DPF through some type of cleaning process.

Ash and soot have a number of fundamental differences, both in terms of their particle size and composition, which affect the manner in which they may be removed from the DPF.  Cleaning processes that involve heating the DPF, so-called “baking” only remove the combustible portion or soot, and do not remove the ash. Since the ash is incombustible, completely different cleaning processes are required to remove the ash.