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DPF: Diesel Particulate Filters

Car exhaust emissionsThe exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the Euro V standard came into force. In fact, many cars registered before 2009 will have had one fitted too in anticipation of the change in standards.

Standards aim to deliver an 80% reduction in diesel particulate (soot) emissions but the technology is not without problems — many drivers are seeing the particulate filter warning light on their dashboard, which indicates a partial blockage of the filter.

Even if your driving isn't mainly urban/stop-start, changes to driving style may be required to keep these systems working properly.

If you're buying a new car and plan to use it mainly for town-based, stop-start driving, you may want to avoid a diesel car fitted with a DPF, because of the possible hassle of incomplete DPF regeneration.

How do DPF filters work?

DPF filters do exactly what they say: they catch bits of soot in the exhaust.

As with any filter, they have to emptied regularly to maintain performance. For a DPF, this process is called regeneration — the collected soot is burnt off at high temperature to leave only a thin ash residue.

Passive and Active Regeneration

Passive Regeneration

Passive regeneration takes place automatically on motorway or fast A-road runs when the exhaust temperature is high. Because many cars don't get this sort of use often, vehicle manufacturers have had to incorporate active regeneration where the engine management system (ECU) takes control of the process.

Active Regeneration

When the amount of soot in the filter reaches a set limit (usually about 45%), the vehicle's ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is too short 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 be possible to complete a regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.

Symptoms of active regeneration

During active regeneration, you may notice one or more of the following:
  • Cooling fans running
  • Increased idle speed
  • Deactivation of automatic start/stop
  • A slight increase in fuel consumption
  • A hot, acrid smell from the exhaust
  • A change in engine note
DPF dashboard warning light DPF dashboard warning light

DPF warning lights

If the regeneration is unsuccessful due to an insufficient driving cycle, the extra fuel injected into the cylinders will not burn and will drain into the sump. As a result, oil quality will deteriorate and the level will rise. Most DPF-equipped engines will have an oil quality/viscosity sensor but it is important that you check that the oil level does not increase above the maximum level on the dipstick as diesel engines can run on their own oil if the level is excessive — often to the point of destruction.

If you ignore the DPF warning light and keep driving in a relatively slow, stop-start pattern, soot will continue to build up until around 75%, at which point you can expect other dashboard warning lights to come on. At this point, driving at speed alone will not be sufficient, and you will need to take the car to a dealer for "forced" regeneration.

Forced Regeneration

Forced regeneration is required when active regeneration criteria have not been met, or where soot levels have increased within the DPF to a point where normal regeneration cannot be performed: typically around 70% soot loading. At this point, the vehicle will enter a restricted performance mode to prevent further damage. If left, the soot loading will continue to rise.

At this level of soot loading, a diagnostic tool must be used to force regeneration. Above around 85% soot loading, regeneration can no longer take place on the vehicle and the DPF will need to be removed for cleaning or replacement.

What can prevent normal regeneration from taking place?

  • Frequent short journeys where the engine does not reach normal operating temperature
  • Wrong oil type — DPF-equipped cards require low ash, low sulphur engine oils
  • A problem with the inlet, fuel or EGR system causing incomplete combustion will increase soot loading
  • A warning light on or diagnostic trouble code logged in the engine management system may prevent active or passive regeneration
  • Low fuel level will prevent active regeneration from taking place. As a rule, a quarter of a tank is required.
  • Oil counter/service interval — exceeding the service interval may prevent regeneration
  • Additive tank low or empty — if the vehicle uses Eolys™ additive, a low level may prevent regeneration

Expensive repairs

If you continue to ignore warnings and soot loading keeps increasing, the car will not run properly and the most likely outcome is that the DPF will need replacing. This costs at least £1,000 plus labour and diagnostic time.

The ash residue which remains after successful regeneration cannot be removed and will eventually fill the filter. DPFs are designed to last in excess of 100,000 miles but, if the vehicle is operated correctly, many will far exceed this mileage.

DPF additives

The most commonly-fitted type of DPF has an integrated oxidising catalytic converter and is located very close to the engine where exhaust gases will still be hot. This heat means that passive regeneration is more likely to be successful.

Some models, across a wide range of manufacturers, use a different type of DPF which relies on a fuel additive (Eolys™ fluid) containing Cerium (III) Oxide. Cerium ignites at a lower temperature and adheres to the soot particles meaning regeneration can occur at a lower temperature.

The additive is stored in a separate tank next to the fuel tank and is automatically mixed with the fuel whenever you fill up. Only very small quantities are used so a litre of additive should treat around 2,800 litres of fuel — enough to cover 25,000 miles at 40mpg. It lasts about 70,000 miles and is replenished during a service (at extra cost).

You will have to pay to get the additive tank refilled at some point in the car's life — expect to pay between £150 and £200 including fluid and labour.

Don't be tempted to ignore a warning light showing that the additive tank needs refilling. It is absolutely essential that this tank is refilled as without it, regeneration is unlikely to be successful and a new DPF may be needed — at significant cost. Fuel consumpution can also increase as a result of failed regenerations.

Check the handbook

If you buy a car with a DPF, it is important that you read the relevant section of the vehicle handbook so that you understand exactly what actions you need to take if the warning light comes on 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.

Removal is not a legal option

It is suggested from time to time that the answer to failed DPF regeneration is to get the DPF removed from the exhaust system rather than pay to get it repaired or replaced. Indeed, there are companies advertising this service including reprogramming the engine management software.

But is it legal?

DPFs are fitted to meet European emissions regulations designed to reduce vehicle emissions or particulate matter (soot) associated with respiratory disease and cancer.

According to the Department for Transport, it is an offence under the Regulation 61A(3) of the Road Vehicles (Construction & Use) Regulations 1986 to use a vehicle which has been modified in such a away 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 regulations, making the vehicle illegal for road use.


You must notify your insurer if the vehicle is modified but such a modification could invalidate any insurance cover because it makes the vehicle illegal for road use.


From February 2014, the inspection of the exhaust system carried out during an MOT test will include a check for the presence of a DPF. A missing DPF, where one was fitted when the vehicle was built, will result in an MOT failure.

With an original equipment DPF removed from the exhaust, the car may or may not pass an MOT smoke test — a Euro V ('59'-plate onwards) diesel with the DPF removed is more likely to fail than a diesel car designed to comply with earlier emissions standards.


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