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Diesel Particulate Filter (DPF)

FAQ - DIESEL PARTICULATE FILTER (DPF) 

What is a DPF? 

A DPF is a clever part of a car’s exhaust which reduces diesel soot emissions by up to 80% The tough exhaust emissions standards introduced in 2009 for new cars effectively required the fitting of a DPF in the exhaust of diesel cars. In fact, many cars registered before 2009 will also have had one fitted too in anticipation of the change in standards. Despite the benefits of a vehicle fitted with a DPF, they can experience issues if the instructions in the vehicle’s handbook are not adhered to. 

Typically these issues are derived from driving which is predominantly urban/stop-start, therefore changes to driving style may be required to keep the DPF working properly. If you are planning to use the vehicle mainly for town-based, stop/start driving it would be wise to seek the advice of local dealer or the manufacturer’s website for advice on suitability. 

How does it work? 

When the soot in the filter reaches a set limit the Engine Control Unit (ECU) will trigger a regeneration process to burn this soot off. If the journey is a bit stop/start or you take your foot off the accelerator while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked. It should generally be possible to start a complete regeneration and clear the warning light by driving for approx 10 minutes at speeds greater than 40mph. However, please note, you should always refer to your particular vehicle’s handbook for vehicle specific information. Expensive repairs - If you ignore the warning light and keep driving in a relatively slow, stop/start pattern, soot loading will continue to build up and you can expect to see other dashboard warning lights start to come on too. At this point driving at speed alone will not be enough and you will have to take the car to a dealer for regeneration. If you continue to ignore warnings and soot loading keeps increasing then the car won’t run properly and the most likely outcome will be that you will have to get a new DPF which could cost a significant amount. 

Check the handbook - If you lease a car with a DPF, it’s important to read the relevant section of the vehicle handbook so that you understand exactly what actions to take if the warning light illuminates and how, if at all, your driving style may need to be adjusted to ensure maximum DPF efficiency and life. In most cases there is only a relatively short time between the DPF being partially blocked and becoming so blocked that it requires manual regeneration. Some vehicles use a special fluid for regeneration the cost of which may not covered by all maintenance contracts. 

Removal is not a legal option. It is suggested from time to time that the answer to failed DPF regeneration is get the DPF removed from the exhaust system rather than pay to get it repaired/renewed. Indeed there are companies advertising just such a service including reprogramming of the engine management software, but is it legal? DPFs are fitted to meet European emissions regulations designed to reduce vehicle emissions of particulate matter (soot) associated with respiratory disease and cancer. According to the Department for Transport, it is an offence under the Road Vehicles (Construction and Use) Regulations 1986 to use a vehicle which has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet. Removal of a DPF will almost invariably contravene these requirements, making the vehicle illegal for road use. MOT From February 2014 the inspection of the exhaust system carried out during the MOT test will include a check for the presence of a DPF. A missing DPF, where one was fitted when the vehicle was built, may result in an MOT failure.

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