<p class="cafe-editor-text">21st Mar 2018<br /><br /><br />If you own a diesel car, you probably have a diesel particulate filter, however, you may not know exactly what this is or how to maintain it.<br /><br />Diesel particulate filters have been fitted to diesel-fuel cars for almost two decades now - but if not maintained, or if tampered with there could be serious consequences for your car.<br /><br />Here we explain exactly what they are, what they do, why you need them and how to look after them.<br /><br />What is a diesel particulate filter?<br /><br />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. But because they only have a finite capacity, this trapped soot periodically has to be emptied or 'burned off' to regenerate the DPF.<br /><br />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.<br /><br />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.<br /><br />READ MORE: What is E10 fuel and how could it affect you?<br /><br />UK Breakdown Cover for £4.50*<br /><br />Rated 4 out of 5 stars on Trustpilot from over 20,000 reviews.<br /><br />Buy now<br /><br />Is it illegal to remove a diesel particulate filter?<br /><br />Yes, it is illegal. Owners face fines if caught (up to £1,000 for cars and £2,500 for vans) and removing a DPF can also invalidate your car insurance policy. <br /><br />Want more useful content like this sent straight to your inbox?<br /><br />Subscribe here<br /><br />How do I tell if my diesel particulate filter is blocked?<br /><br />If the DPF is becoming clogged with soot or a fault develops in the system, an orange light will typically appear on the dashboard as seen below.<br /><br /><br /><br />They usually look like this with a piped box that has dots in the middle, although they can slightly vary by manufacturer manufacturer – check your handbook for more information.<br /><br />What causes a diesel particulate filter blockage?<br /><br />Short journeys at low speeds are the prime cause of blocked diesel particulate filters.<br /><br />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).<br /><br />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 100,000 miles.<br /><br />It’s important you use the right type of oil as well – some oils contain additives that can actually block filters.<br /><br />READ MORE: What is a Low Emission Zone - everything you need to know<br /><br />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.<br /><br />How do I maintain a diesel particulate filter?<br /><br />The best way to maintain a DPF is to make sure it’s fully able to regenerate itself when it’s full of soot (when the warning light appears).<br /><br />There are two types of regeneration: passive and active.<br /><br />Passive regeneration <br /><br />Passive regeneration occurs when the car is running at speed on long motorway journeys which allows the exhaust temperature to increase to a higher level and cleanly burn off the excess soot in the filter.<br /><br />So it is advised that drivers regularly give their diesel vehicle a good 30 to 50 minute run at sustained speed on a motorway or A-road to help clear the filter.<br /><br />However, not all drivers do this type of driving regularly – which is why manufacturers have designed an alternative form of regeneration.<br /><br />Active regeneration <br /><br />Active regeneration means extra fuel is injected automatically, as part of the vehicle's ECU, when a filter reaches a predetermined limit (normally about 45%) to raise the temperature of the exhaust and burn off the stored soot.<br /><br />Problems can occur, however, if the journey is too short, as the regeneration process may not complete fully.<br /><br />If this is the case the warning light will continue to show the filter is still partially blocked. <br /><br />In which case it should be possible to complete a regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.<br /><br />You will know whether active regeneration is taking place by the following symptoms:<br /><br />Engine note changeCooling fans runningA slight increase in fuel consumptionIncreased idle speedDeactivation of automatic Stop/StartA hot, acrid smell from the exhaust<br /><br />What do I do if neither active nor passive regeneration work?<br /><br />If your warning light continues to stay on, turns red, or additional DPF lights come on, do not leave it too long before getting it checked out.<br /><br />More damage can be caused this way and what could be an inexpensive fix can become something much more expensive.<br /><br />Some garages can clean blocked DPFs, in a process called forced regeneration.<br /><br />This usually costs around £100 and, while it’s not a 100% guaranteed fix, it’s usually successful in removing the excess soot and allowing the DPF to work and automatically regenerate again.<br /><br />It’s a failure to correctly regenerate that is the cause of most diesel particulate filter issues: they become blocked, which increases exhaust emissions, stifles engine performance and sometimes even puts the car into a restricted ‘limp-home mode’.<br /><br />On some models the engine may not restart after a number of miles – again, consult your handbook for details.<br /><br />MORE ADVICE: What is LPG?<br /><br />Modern diesel car owners thus need to be conscious of the importance of maintaining their diesel particulate filter through driving habits and practices.<br /><br />Do I need a diesel particulate filter to pass the MOT?<br /><br />A diesel particulate filter check has been part of the MOT test since February 2014. If a filter has been removed, the car will fail its MOT.<br /><br />Removing the DPF will sometimes cause the warning light to glow – and this itself is an MOT failure point: no dashboard warning lights should remain on during the test.<br /><br />Should I call the RAC if the diesel particulate filter light is glowing?<br /><br />There is no need to call out the RAC if your DPF warning light is glowing. As mentioned, it can sometimes be rectified by going on a long drive; if it fails to extinguish, take your car to a garage.<br /><br />Remember, orange warning lights are advisory; it’s red warning lights that require immediate attention and usually demand you stop the car as soon as possible.<br /><br />READ NEXT: The real facts on 'dirty' diesels<br /><br />What is the cost of a new diesel particulate filter?<br /><br />Diesel particulate filters are very expensive. A new one from a car manufacturer can cost £1,000 and £3,500, potentially wiping out the cost savings associated with driving a diesel.<br /><br />As cars age, the cost of the replacement DPF could be more than the value of the car – and it is older, higher mileage cars that are most likely to require a new DPF.<br /><br />There are now other suppliers of diesel particulate filters that charge less, but be careful here: they must still have the correct Type Approval or they may not work correctly and end up costing you more in repairs.<br /><br />Want more useful content like this sent straight to your inbox?<br /><br />Subscribe here<br /><br />5 Star RAC Car Insurance Plus<br /><br />Get cover for less than £165 (that’s what 10% of our customers paid) **<br /><br />Get a quote<br /><br />Emissions<br /><br />ShareFacebookTwitterGoogle PlusEmail<br /><br />*New, Vehicle based Roadside only on a monthly auto renewing contract<br /><br />** 10% of all customers buying Car Insurance Plus with us from 1st January 2018 to 30th June 2018 paid less than £165. 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