Diesel particulate filters

If you drive a diesel car which was manufactured after 2009, it will be fitted with its own diesel particulate filter (DPF). Some models will have introduced DPFs even earlier, specifically those designed to Euro 5 emissions standards.

Introduced by car manufacturers to aid in the reduction of exhaust emissions, these filters capture and store the excess exhaust soot produced by your car. However, with a finite capacity, blockages are common and soot does have to be occasionally ‘burned off’ or manually emptied in order to regenerate the DPF and help prevent the hassle of a potential breakdown and expensive repair costs.

However, how do you actually know when your car’s diesel particulate filter needs some attention and what is the best method for maintaining this vital component? Here’s everything you need to know about DPFs: what they are, how they work, and how best to look after them.

What is a DPF filter?

Unlike petrol engines, the compression-ignition combustion process involved with powering diesel engines results in the creation of a sooty by-product. Invisible particles contained within this soot can cause significant health and environmental problems if they leave the car as untreated exhaust fumes. The job of a diesel particulate filter is to trap this soot, and therefore any harmful particles, and prevent them from being pumped out into the atmosphere in the form of the tell-tale smokey black exhaust fumes associated with older diesel vehicles. 

In an attempt to help lower CO2 emissions produced by vehicles within Europe, Euro 5 exhaust emissions legislation was introduced by the European Union (EU) in 2009. This legislation saw DPFs become a mandatory component of all new diesel cars manufactured in the EU. 

Removal of the DPF is now illegal and is actually checked as part of your vehicles MOT. If you are found to be driving a vehicle without a diesel particulate filter (where a DPF was originally fitted) the penalty fine is up to £1,000 for cars and £2,500 for light goods vehicles.

How do DPFs work?

The inside of a DPF is usually made up of a highly porous silicon carbide honeycomb. When the unfiltered diesel exhaust fumes flow through the channels of the DPF, the porous walls of this catalytic honeycomb structure trap solid harmful particles and allow the filtered gaseous components of the exhaust to escape in vapour form. 

While this process does prevent the vast majority of harmful particles from being released into the atmosphere, the soot itself forms a ‘cake’ layer inside the filter. An excess of this cake layer can lead to blockages when the diesel particulate filter is full, and this can cause potential breakdowns. 

How diesel particulate filters regenerate

To prevent a blockage from occurring, it is essential that your DPF is kept as clean as possible. In order to do this, excess soot stored inside the filter needs to be burned at a high enough temperature to turn into finer ash particles which can then be expelled from the system. This self-cleaning process is known as ‘regeneration’. If this process is interrupted and the DPF is not able to regenerate, faults can occur with the exhaust system.

Types of DPF regeneration

There are two types of regeneration, depending on the system fitted by the manufacturer: active and passive.

Active regeneration occurs when extra fuel is automatically injected by the car’s Electronic Control Unit (ECU). This acts to raise the temperature of the exhaust produced by the car, which in turn burns off the excess soot stored inside the DPF.

Passive regeneration, on the other hand, occurs at higher speeds when an engine is running at higher revs. Most car manufacturers recommend driving at a consistent speed of over 40mph for a period of more than 30 minutes every few hundred miles to help prevent any DPF blockages. Regular motorway drivers will do this without even realising. 

The regeneration process always leaves a small amount of ash residue in the filter which builds up over time and gradually blocks the pores in the walls of the DPF. This means it has to be replaced approximately every 50,000 – 80,000 miles.

Do DPF cleaners work?

While it is claimed that DPF cleaners can work to help prevent the build-up of soot and potential blockages, it is generally accepted that the best and most effective way of reducing potential problems with your exhaust system is encouraging your car’s inbuilt DPF regeneration processes.

Cleaning your DPF

You can encourage the DPF regeneration process and prevent DPF blockages by undertaking regular long trips in which you keep the engine revs between 2,000 and 3,000rpm for at least 30 minutes.

DPFs can be professionally inspected and cleaned if need be. However, this requires removal of the unit and specialist equipment to assess and test after cleaning to ensure it is successful. Cleaning of this type is typically 90-95% effective, compared to a replacement unit.

Exhaust after-treatment

Some vehicles rely on a fuel treatment known as EOLYS, which is injected into the fuel tank and lowers the temperature needed for regeneration to happen – this fluid reservoir needs refilling (or replacing) which can be completed at Kwik-Fit.

AdBlue™ is another fluid that is common on diesel vehicles, but this does not directly impact the DPF – however it is important to keep that topped up to prevent other issues.

Diesel particulate filters at Kwik Fit

Many DPF issues are actually caused by other faults on the vehicle, or by poor service and maintenance.  Your vehicles engine oil is critical to the operation of the DPF, and use of poor quality oil, or failing to have your vehicle serviced can increase the likelihood of DPF issues.

Your car’s onboard computer system will monitor the state of your DPF and if it starts to sense the filter is becoming partially blocked, a DPF warning light will appear on your vehicle’s dashboard. If this light stays illuminated for a number of days, turns red or starts to beep or flash, it is likely that attempts at regeneration have failed and you should take your car to your nearest Kwik Fit centre to be checked out as soon as possible.

DPF cleaning and repairs

At Kwik Fit, our master and diagnostic technicians can diagnose your DPF issues and carry out the appropriate repairs. If your DPF warning light appears and is overlooked, this will usually be followed by symptoms such as a loss of power, poor fuel economy, cooling fans running more than usual, and the automatic stop-start system not working in your vehicle.

Don’t delay – contact your nearest Kwik Fit centre for assistance.