The diesel particulate filter: How a DPF works and why it can cost you a fortune

Lewis Kingston

31 Jan 2022

1/5
Thinking about buying a diesel car and curious or concerned about diesel particulate filters? Find out how they work, and what the pitfalls are, in this YesAuto guide

If you’re shopping around for a diesel-powered vehicle, you may have encountered adverts, reviews or technical specifications that mention diesel particulate filters.


These diesel particulate filters – DPFs – form an essential and very effective part of modern emissions-reducing systems.


However, the way they work can cause problems for some owners. Left unchecked, those issues can subsequently turn into big bills. DPFs don’t last forever, either, and replacements are expensive.


Consequently, if you’re in the market for a diesel car or work vehicle, read on to find out more about diesel particulate filters – including how they work and what the potential pitfalls and costs can be.


What is a diesel particulate filter (DPF)?


Diesel engines, when running, emit a variety of particulate matter – including soot and other fine particles – that is harmful to both humans and the environment.


A diesel particulate filter, as the name suggests, filters particulate matter out of the engine’s exhaust stream, reducing environmental and health-related impacts. 


How does a diesel particulate filter work?


A diesel particulate filter typically consists of a ceramic core that is mounted in a metal housing.


Inside the core, there are specially designed passages or porous structures. These capture particulate matter – soot from combustion, primarily – while the exhaust gas flows through the DPF.


When the filter gets hot enough, either naturally or through artificial means, the stored soot and particulates are burnt off. This process is called regeneration and prevents clogging.


The reaction that occurs during regeneration – often permitted or aided by special coatings, catalytic elements or an injected additive – converts the carbon soot into less harmful carbon dioxide.


This then flows out of the exhaust system, along with the other waste gases from combustion.



Do all diesel-powered cars have DPFs?


The Euro V emissions standard, introduced in September 2009, included tighter legislation on engine particulate emissions.


The new standards effectively made DPFs mandatory for diesel engines in the UK, although some cars were already equipped with them by that point.


DPFs are also found in other vehicles, including pick-up trucks, vans and heavy commercial and construction equipment. 


What’s the difference between passive and active regeneration?


Passive regeneration is when stored particulates in the DPF are burnt off naturally. This often happens when exhaust temperatures are higher, such as when the car is cruising at motorway speeds. 


When the car is being driven slowly, or the engine load is low, however, the DPF temperature often won’t get high enough for passive regeneration to occur.


This is when an active regeneration cycle, triggered by sensors monitoring the capacity and flow of the DPF, is called for.


The active regeneration cycle will alter the engine and injection system’s behaviour to artificially raise the exhaust gas temperature, allowing soot to burn off and making the DPF ready for use again.


How do I know active regeneration is taking place?


Signs of an active regeneration typically include a raised engine idle speed, cooling fans running, an odd smell, a high engine bay temperature and increased fuel consumption.


A warning light may also be displayed, indicating that a regeneration process is occurring or required.


If an active regeneration cycle is taking place, try to avoid interrupting it and continue driving until normal operation resumes.


If you are unsure what to expect or do, or have questions about a specific warning message, consult your vehicle’s manual.



What happens if a DPF gets blocked?


If a DPF isn’t given the chance to regenerate, or is suffering due to another issue, and warnings are ignored, the filter may get clogged.


Depending on the severity of the accumulation in the DPF, engine power may be reduced, starting might prove problematic, regenerations won’t be possible, and additional warnings may be displayed.


Fortunately, a process called forced regeneration may help clear the filter out and allow for normal operation and regenerations to resume.


DPF issues are often caused by a problem elsewhere, though, so it’s essential to get everything else checked to avoid the problem repeating.  


In any case, try to avoid letting a DPF get into such a state – as it could ultimately end up requiring removal and cleaning, or replacing completely.


How much does a new DPF cost?


Diesel particulate filters are expensive. A name-brand DPF for a small diesel-powered hatchback can easily cost more than £500, for example, whereas one for a larger and more upmarket car might cost north of £1,000.


That’s before you factor in labour and other checks and supporting parts, too. Consequently, it’s worth getting a quote for DPF replacement before jumping into diesel power with both feet.


Can a diesel particulate filter be cleaned?


The good news is that, provided the filter is undamaged, a DPF can be removed and professionally cleaned to remove accumulated soot – and doing so is often far less expensive than a new filter assembly.


Some offer cleaning that doesn’t involve removing the DPF from the car, but such services should be researched carefully. Online reviews and forum posts may help point you in the right direction.   



Can’t I just remove the DPF?


Removing a DPF will invariably mean that the vehicle can no longer meet emissions standards, making it technically illegal for use on the road.


A missing DPF should also result in an MOT test failure, as will one that has been visibly tampered with. It may have been cut open, for example, to remove its filtering core, and then welded shut.


If the filter was opened for cleaning and there is evidence to support this, however, it won’t be deemed a failure point.


Do diesel particulate filters wear out?


Not everything can be burnt off by the DPF and, over time, incombustible material will accumulate as ash in the filter assembly.


As the ash builds up, the soot capacity and capabilities of the filter will degrade. Regenerations will increase, as will fuel consumption, and the filter may block more easily.


Eventually, the DPF will require professional cleaning or replacement.


How long will a diesel particulate filter last?


Compliance with regulations and durability testing means that DPFs should – and often do – last in excess of 100,000 miles without much effort.


However, how long a DPF lasts is very dependent on how a car is driven, the type of journeys it undertakes and how well it is maintained.


A lot of short journeys will potentially shorten the life of a DPF, whereas a car that covers long distances on the motorway could have a particularly long-lived DPF.



How can I prolong the life of my DPF?


To help ensure that a DPF has a long service life, ensure that it is given adequate opportunity to regenerate.


Ideally, you want to avoid using the car solely for short trips. At the very least, take it for a longer and higher-speed run occasionally.


For those commuting or travelling longer distances regularly, DPF regeneration shouldn’t prove problematic.


What is also essential is using the correct low-ash oil, also known as low sulphur, sulphate ash, phosphorous and sulphur oil – low SAPS oil – which reduces substance build-up in the DPF.


Similarly, using high-quality diesel, even just the odd tank or two from time to time, can reduce soot production and deliver running and efficiency improvements.


Off-the-shelf fuel additives are also available to help clean and maintain DPFs. The quality and capability of such additives may vary, however, so research them carefully.


Lastly, don’t forget to consult your vehicle’s manual. At the very least, it should explain to you what warning messages might arise and the best course of action to take.


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Lewis Kingston

31 Jan 2022