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DPFs & DPF Cleaner Explained

Diesel particulate filters (DPF) have been a mandatory component of diesel cars since 2009, but they can degrade over time and eventually even begin to cause damage to your vehicle if not properly maintained. This article gives the lowdown on what a diesel particulate filter is, what a DPF cleaner is, and why you should stay on top of DPF cleaning.

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What is a diesel particulate filter (DPF)?

A DPF is a filter that captures and stores exhaust soot to reduce diesel car emissions. However, due to their finite capacity, the trapped soot has to be emptied periodically or ‘burned off’ to regenerate the DPF from a blocked state.

The regeneration process burns off the excess soot in the filter, reducing the harmful emission and preventing the tell-tale black smoke that previously typified diesel vehicles.

The Euro 5 exhaust emissions legislation (2009) helped lower car CO2 emissions and made DPFs mandatory. Since then, approximately 50% of new cars released per annum are diesel-powered.

What causes a DPF blockage?

There are several factors that can contribute towards a DPF blockage. Short journeys at low speeds are potentially the primary cause, which is why city drivers are generally recommended petrol cars over diesels. Poor servicing standards are another common factor, reinforcing the importance of taking your car to a reputable garage. Finally, the use of low-quality fuel may impede DPF regeneration to save energy, which can cause severe damage to the DPF.

How to maintain a DPF

The best way to maintain a DPF is to ensure it can regenerate even when it's full of soot. The two types of regeneration are explained below:

Passive DPF regeneration

Passive regeneration occurs when the car runs at speed on long motorway journeys, which increases the exhaust temperature and cleanly burn off the excess soot in the filter. Therefore, drivers are advised to give their diesel car regular 30–50-minute runs at a sustained speed on a motorway to clear the filter. Not all drivers do this, however, prompting car makers to design an alternative form of regeneration.

Active DPF regeneration

Active regeneration injects extra fuel automatically when the filter reaches a predetermined limit (usually about 45%). This raises the exhaust's temperature and burns off the stored soot. However, problems can occur if the journey is too short, as the regeneration process may not finish.

A dashboard warning light will illuminate in such instances, indicating the filter remains partially blocked. Driving for 15 minutes or so at greater speeds than 40mph should complete the regeneration cycle and deactivate the warning light.

The following symptoms indicate active regeneration:

  • A hot, acrid smell from the exhaust
  • Engine note change
  • Cooling fans running
  • A slight increase in fuel consumption
  • Increased idle speed
  • Deactivation of automatic Stop/Start


What to do if active and passive regeneration fail

If the DPF dashboard warning light stays on or turns red, it's critical to have your car checked by a professional.

Failing to do so could cause more damage and sharply increase repair costs. Some garages clean blocked DPFs in a process called forced regeneration, costing around £100. While it's not guaranteed to work, it's usually successful in removing the excess soot and enables the DPF to regenerate again automatically.

Using DPF cleaner to aid regeneration

The purpose of DPF cleaner is to lower the temperature at which the diesel particulate filter can regenerate (or burn off the soot that’s blocking it). By allowing the DPF to start regenerating at a lower temperature, DPF cleaner assists with the timely removal of particulates that are clogging up the filter.

When it’s used regularly as part of a car maintenance routine, it can help to keep your DPF in good condition for longer. You’ll also be able to avoid the need to regularly take longer or faster trips to spur on passive regeneration, minimising fuel wastage.

How to use DPF cleaner

While the specific usage instructions will vary from one DPF cleaner to another, it’s generally used by adding a certain ratio of the liquid to your fuel tank. It’s generally not necessary to use DPF cleaner every time you refuel, with most products recommending use every second or third tank filling.

Once you’ve added the DPF cleaner to your tank, it’s usually recommended that you drive for at least another 15 minutes, preferably on a road with a fairly high speed limit. This will give the cleaner time to get to work on your blocked DPF, burning off any excess soot.


How much do new DPFs cost?

DPFs are very expensive, with new ones costing between £1,000 and £3,500, potentially wiping out the cost savings associated with diesel cars. As cars age, the cost of the replacement DPF could exceed the car's value, and it’s older, higher mileage cars that are more likely to need a new PDF.

Is it illegal to remove a DPF?

Yes, it is illegal. If you remove your vehicle's DPF, you could face a fine of up to £1,000 for a car or £2,500 for a van. Removing a DPF can also invalidate your car insurance policy.

How do I tell if my diesel particulate filter is blocked?

If the DPF clogs with soot or a fault develops in the system, an orange light will typically appear on your car’s dashboard.

Do I need a DPF to pass an MOT?

Yes, your diesel car must have a DPF to pass its MOT. DPF checks have been part of the MOT since February 2014, and cars that have had their DPFs removed will automatically fail the test.

What should I do if the DPF light comes on?

Orange warning lights are advisory; red warning lights require immediate attention and demand you stop the car as soon as possible.