Diesel engines produce a lot of particulate matter (soot), this can cause respiratory problems when breathed in and has also been linked to cardiovascular disease. To reduce these types of emissions Diesel Particulate Filters (DPF) have been developed. DPFs are devices that physically capture the particulates created by the combustion of diesel and prevent their release to the atmosphere. Specific materials have been developed that can remove 80-90% of the particulates. DPF filters are most effective in capturing solid diesel particulates, including carbon (soot) and the related black smoke emission and these filters have become the leading technology for reducing particulate emissions in diesel engines. Since 2009, when the ‘Euro 5’ standard came into use, it has been a requirement that all new diesel vehicles be fitted with a DPF, and since February 2014 the following update to MOT protocols in the UK has been in force.

a lot of exhaust fumes in a traffic jam

“Garages and testing stations will be required to check for a diesel particulate filter (DPF) in the inspection of the exhaust system as part of the MOT test (or annual test for heavy vehicles) from February 2014.

The vehicle will automatically fail the MOT test if the filter has been fitted as standard but is found to be no longer present.”

DPF Regeneration – Active & Passive

The particulate captured by the DPF will build up over time, and the filter requires emptying to remove the build-up. This is usually a function carried out passively during normal usage of the vehicle. When the vehicle reaches the required speed and engine revs to heat the exhaust temperature to the required point the collected soot is burnt off leaving a small and harmless ash residue. The ash cannot be removed without the DPF being removed and cleaned using specialist techniques but, when maintained correctly, a DPF should function well for over 100,000 miles.

A diesel particulate filter seen from below

Passive regeneration is only possible, as discussed, when the exhaust reaches the required temperature. This only occurs when you drive for an extended period at the required speed. This is the principal reason why diesel cars may be unsuitable for drivers who make frequent short journeys around town, rather than using A roads and Motorways. The DPF needs the exhaust to reach around 500 degrees Celsius to begin passive regeneration, and typically you can initiate passive regeneration of the DPF by driving at around 40 mph for around 10 minutes. If this isn’t something that you do regularly, then it is likely that the soot will build up which can cause problems like reduced fuel-efficiency and misfiring.

Many vehicles have a function for ‘Active Regeneration’ of the DPF. This is a function which begins automatically if the DPF has not been cleared by passive regeneration. During active regeneration the engine will let the exhaust gases run hot, even when not travelling at speed, to increase the exhaust temperature and burn off the particulate. The frequency with which your vehicle will actively regenerate the DPF is dependent on the model. This is a good thing to find out before buying or as the owner of a diesel car. Also, as active regeneration can only take place when the vehicle is moving, if you use your vehicle most frequently in stop-start urban traffic your vehicle may be unable to actively regenerate the DPF.

Active regeneration should take place every 300 miles or so, though this can depend on the way that you use your vehicle. On most vehicles it will take between 5 and 10 minutes to complete the process. If your journey’s too short, then the regeneration process won’t finish and the DPF will not be successfully cleared. The normal regeneration processes can also be prevented by the use of incorrect engine oils. If your vehicle is fitted with a DPF it is strongly recommended that you use the type of engine oil specified by the manufacturer. Most manufacturers specify a low SAPS oil which is specifically designed to reduce the production of Sulphated Ash which is a significant contributor to the build-up in the filter mesh of your DPF.

DPF Warning Light Explained

A warning light letting you know your DPF is blocked should not be ignored, but can in many instances be remedied by driving in a way which initiates passive/active regeneration of the DPF. Driving for over 10 minutes at speeds in excess of 40mph will often be sufficient to clear the blockage and regenerate the DPF. You should consult the handbook for your vehicle for more specific guidance on how you should drive to initiate regeneration of the DPF and clear the warning light. Nevertheless, a general rule of thumb to follow is that you should run the engine at least 40 mph and at least 2500 rpm.

Ignoring the DPF light, or failing to clear the blockage in a timely manner, will lead to continued soot build up in the DPF. This will cause your car to go into what is known as ‘restrictive performance mode’, which is designed to protect your vehicle from further damage. If things have progressed to this point you will need to have your vehicle looked at professionally.

DPF Cleaning

an exhaust from a diesel car

The first option available to a professional is ‘Forced Regeneration’. This is a method of cleaning the DPF using a manufacturer designed maintenance. The right tools are required, along with access to the vehicles management systems, so this is not something that you can do yourself. This will involve the mechanic running a DPF clean cycle which runs the car at a high RPM for a prolonged time. The ECU makes this cycle heat up the DPF filter to high temperatures, clearing the filter by burning its contents.

Alternatively, a manual DPF cleaning process to clear the filter can be carried out. This uses ultrasonic technology and specialist chemicals, and will return the DPF to near new condition. With the cost of replacing a DPF unit reaching £2,000 in some instances, having your DPF cleaned by a professional is certainly a viable alternative.

DPF cleaning additives, which can reduce the temperature required for soot to burn off, can in some limited situations help your vehicle achieve regeneration. They are by no means guaranteed however, particularly if the filter is blocked with ash as opposed to soot. We recommend that you seek a professional assessment first, but if you are presented with the cost of a forced regeneration, professional clean, or replacement, it can be worth trying this route first. Particularly if your vehicle was produced between 2011 and 2014 and the DPF is located further from the engine than is now standard.