DPF Filters, What Are They? And How Do They Work?

Most modern Diesel cars now come with a DPF, but what does this mean for you?

Exhaust emissions standards introduced by Euro 5 in 2009 mean that all new diesel cars sold since then (and some before) have been fitted with a DPF (diesel particulate filter). The DPF helps to reduce the emissions of harmful particulates such as soot by up to 80 per cent. However, the technology has been controversial, with some car owners claiming that they have been faced with hugely expensive repairs bills when the DPF goes wrong and roadside assistance regularly called out to vehicles experiencing a partial blockage of the DPF.

The efficient functioning of the DPF can be affected by your driving habits. A DPF can mean that diesel cars are not necessarily the best option for those living and mostly driving in urban environments and making lots of short, stop-start journeys.

The DPF is essentially a trapping mechanism which captures soot as it travels through the car’s exhaust system and stops it being released into the air. Like all filters the DPF needs to be cleaned on a regular basis, a process called regeneration. This is carried out by burning the soot off the filter at high temperatures, leaving a tiny residue of ash behind.

DPF (Diesel Particulate Filter) Regeneration

There are two forms of regeneration used by modern diesel cars: passive and active. In passive regeneration, the process is carried out automatically when the car is driven at higher speeds such as on motorways or faster A roads. However, as many cars rarely make this kind of journey, manufacturers have had to develop an active form of regeneration, a task designated to the car’s ECU (engine management system).

Active regeneration occurs when the ECU senses that the filter has reached a specified limit, such as 45 per cent of its capacity. The system then begins a post combustion process of fuel injection which raises the temperature of the exhaust system to force regeneration to take place.

Regeneration Interruption

However this system can also be problematic as active regeneration takes time to complete. If the car comes to a stop during the regeneration process the cycle fails. This can cause a DPF warning light to be illuminated on the car’s dashboard indicating a partial blockage of the filter. Similarly using the wrong engine oil or driving with insufficient fuel can have the same effect. More modern diesel cars may include a special additive to facilitate regeneration, so you need to keep an eye on the level of the additive tank.

Incomplete regeneration cycles can cause the deterioration of engine oil, as the unused fuel injected to start the process drains off into the sump. If you notice an increase in your oil level or receive an oil warning, DPF regeneration interruption could be the cause.

You can manually stimulate the regeneration process by driving at a minimum speed of 40mph for about ten minutes, to give the mechanism time to complete its cycle and clear the DPF warning light. Alternatively, drive with the revs at least at 2000rpm for 5-10 minutes to produce a higher running temperature.

Don’t Ignore Warning Lights

What does a DPF warning light look like? This can be found in your cars manual. Here is a picture of a DPF warning light. Some people mistake them for an EML (Engine managment light) Which on ocassions both can be illumintaed.


A failure to respond to the DPF warning light means that the filter will continue to collect soot. When it reaches about 75 per cent, other warning lights will appear on the dashboard and the car’s performance will be restricted. At this point a more complicated unblocking process will be necessary to clear the filter, which must be carried out by a mechanic. Should the filter reach a capacity of about 85 per cent, the filter will need to be removed for manual cleaning or replaced altogether, which can cost £1,000 or more.

Most DPFs should be good for 100,000 miles of motoring but, this is entirley down to driving style, short trips, long trips, and fuel.

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