"A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine."

DPFs reduce diesel soot emissions by 80% but they're not suitable for everyone.

The exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the 'Euro 5' 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's not without problems – AA patrols are regularly called to cars with the particulate filter warning light on indicating 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 it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF) because of the possible hassle of incomplete 'DPF regeneration'.

DPF warning light - How do they work?

Diesel Particulate filters (DPF) or 'traps' do just that, they catch bits of soot in the exhaust.

As with any filter they have to be 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 tiny ash residue.

Regeneration is either passive or active

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 vehicle manufacturers have had to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Active regeneration

When the soot loading in the filter reaches a set limit (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 the following symptoms:

Cooling fans running

Increased idle speed

Deactivation of automatic Stop/Start

A slight increase in fuel consumption

A hot, acrid smell from the exhaust.

Engine note change

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 loading will continue to build up until around 75% when you can expect to see other dashboard warning lights come on too. At this point driving at speed alone will not be enough and you will need to take the car to a dealer for 'forced' regeneration.

Forced regeneration

Forced regeneration is required where `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 keep rising.

At this level of soot loading a diagnostic tool must be used to force regeneration. Above around 85% soot loading regeneration can no longer be performed on the vehicle and the DPF will need removing to be cleaned or replaced.

What can prevent normal regeneration taking place?

Frequent short journeys where the engine does not reach normal operating temperature

Wrong oil type - DPF equipped cars require low ash, low sulphur engine oils

A problem with the inlet, fuel or Exhaust Gas Recirculation (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 catalyst regeneration

Low fuel level will prevent active regeneration taking place. As a general rule ¼ 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 then the car won’t run properly and the most likely outcome will be that you will have to get a new DPF costing at least £1000 plus labor 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 2800 litres of fuel – enough to cover 25,000 miles at 40mpg. It lasts about 70000 miles and is replenished during a service – at extra cost.

You will have to pay to get the additive tank refilled at some time 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 tanks need refilling. It's absolutely essential this tank is refilled as without it regeneration is unlikely to be successful and a new DPF may be needed – at significant cost. Fuel consumption can increase as a result of failed regenerations too.

Check the handbook

If you buy 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.

We're seeing some evidence of DPF systems failing to regenerate even on cars used mainly on motorways.

On cars with a very high sixth gear the engine revs may be too low to generate sufficient exhaust temperature for regeneration. Occasional driving in a lower gear to maintain around 2,000rpm should be sufficient to burn off the soot in such cases (refer to the vehicle handbook).

DPF regeneration will be initiated by the ECU every 300 miles or so depending on vehicle use and will take 5 to 10 minutes to complete. You may notice other symptoms and a puff of blue smoke from the exhaust, similar to an engine burning oil, when the process is complete.

There's no evidence in AA breakdown data that the problem's going away – newer car models seem just as likely to suffer DPF problems if not driven 'correctly' as those built when DPF's were introduced.

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 (Regulation 61a(3)) 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.


You must notify your insurer if the vehicle is modified but such a modification could in turn 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 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, 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 (September 2009 diesel) is more likely to fail than one designed to comply with earlier emissions standards.

1. What is a DPF Filter? And why does it need to be cleaned?
2. How often should my DPF be cleaned?
3. What benefits do I get with regular DPF cleaning?
4. How do I know the DPF needs cleaned?
5. What is the DPF Deep Clean process?
6. How effective is the cleaning process from DPF Deep Clean?
7. How long does it take to clean using your process?
8. Is the cleaning process safe for my DPF unit?
9. I have a catalytic converter and DPF unit can they both be cleaned?
10. Can you clean the EGR Valve?
11. The DPF is a single unit that cannot be disassembled, can DPF Deep Clean process it?
12. If I had a catastrophic engine failure, can my DPF be cleaned?
13. My DPF is completely blocked, can it be cleaned?
14. Is there any water left in the DPF after cleaning?
15. How do I know if my DPF unit is damaged (Cracked)?
16. How do DPF Deep Clean verify the results of cleaning?
17. Can DPF Deep Clean remove and refit the filter unit?

1. What is a DPF Filter? And why does it need to be cleaned?

Diesel particulate filters (DPF'S) are emission control devices designed to clean the exhaust emitted by the vehicles' diesel engines. In a diesel particulate filter (DPF), exhaust gasses pass through the open end of a channel and at the opposite end is a plug that forces the gasses through the porous channel wall allowing the exhaust to escape through the neighboring channel wall trapping as much as 90 percent of the solid particulate matter.

The most popular type of filter is a “wall flow” filter which can remove between 90-100% of the particulate matter from the exhaust under proper operating conditions. Wall flow units typically consist of thousands of small (<1mm) alternately blocked rectangular channels in a ceramic core. 50% of the rectangular channels are inlet channels, in which ash/soot will accumulate and 50% are outlet channels, which under normal conditions remain clear. The thickness and porosity of the walls determine the efficiency of the unit filter and it is on the inlet side surfaces of the wall that the ash/soot collects. Ash/Soot collects at the outlet end of the filter channels and filter efficiency decreases over time until the unit becomes non-serviceable. Regenerative cycles are used intermediately to extend the DPF working life by combusting the accumulating soot to carbon dioxide and ash. Some DPF units utilize an oxidative fuel additive during the regeneration cycle to help promote the combustion of the soot in the unit. Some units simply heat and air to provide the necessary energy and some DPF cores are enhanced with catalyst to improve the regenerative performance. Regardless of the frequency and effectiveness of the regeneration, eventually the DPF will become fouled to the point that it is no longer operative, at this point the unit must be replaced or cleaned by another method.

2. How often should my DPF be cleaned?

Unfortunately many vehicle dealers imply that all that is needed to keep the DPF clean is to hit the vehicle's “regeneration” button. While it's true that the regeneration cycle is necessary and will combust much of the accumulated particulate matter, non-combustible materials trapped in the filter will still be left behind, this remaining residue creates back pressure that will eventually lead to increased regeneration downtime, loss of horsepower, reduced fuel economy, filter damage and ultimately engine damage. The frequency of the DPF cleaning will vary by vehicle usage, in extreme cases the engine control will significantly reduce engine output or shut down completely should the exhaust back pressure caused by the DPF exceed critical levels. Recommendations for DPF service (removal of the DPF and cleaning by a specialized process) vary by manufacture and vary greatly with vehicle usage. The typical schedule would require DPF service every 250,000 km for light to medium duty operation, for smaller passenger and light duty diesel vehicles, DPF service may be required at much shorter intervals.

3. What benefits do I get with regular DPF cleaning?

DPF cleaning will normally restore engine performance, increase fuel efficiency, reduce regeneration, extend filter life and overall reduce down time and maintenance costs.

4. How do I know the DPF needs cleaned?

Increased frequency of regeneration cycles and loss of engine performance are the best indicators of the need for DPF cleaning. If you are operative under light load or frequent idle conditions, or if you notice a significant increase in regeneration cycles it is likely time for a DPF service. DPF Deep Clean's unique process can have your DPF cleaned in usually less than a working day meaning you are back on the road faster.

5. What is the DPF Deep Clean process?

DPF Deep Clean has developed a 9 stage process which involves a non-destructive proprietary cleaning process, a proprietary cleaning solution and high volume airflow to agitate and assist in the complete removal of ash/soot from the channels of the DPF unit. We will inspect the DPF on arrival and we will ask for some information about the reasons for failure. We will start by inspecting the DPF with our endoscopic equipment for signs of cell damage and oil damage, any units found damaged as a result of catastrophic engine failure may not be repairable and we will try to determine this at the outset of the cleaning activity. Through testing we determine if the unit is lightly, normally or heavily fouled and may adjust the return expectations based on this. We will weigh the DPF and airflow test for back pressure results, we will then place the DPF into the proprietary cleaning process where we agitate the ash/soot plugs which become saturated with liquid and softened. The ash/soot plugs are then washed out with low pressure rinse water and given a final rinse with fresh water. We then thoroughly dry the DPF and repeat the pre inspection of airflow for back pressure, the DPF is re-weighed and undergoes a final endoscopic inspection. Having confirmed that the cleaning has removed all of the ash/soot contamination, the DPF is then returned along with the paperwork showing pre-inspection and final inspection results.

6. How effective is the cleaning process from DPF Deep Clean?

Results from both our R&D testing and in-house process validation checks have shown between 96-100% of accumulated ash/soot in a DPF are removed, the DPF returned to an “as new” condition and the back pressure generated by the filter is restored to factory specifications.

7. How long does it take to clean using your process?

The typical time to turnaround a DPF is 4-6 hours. This time may increase if we have a large number of DPSs in process, but we can let you know when the DPF will be ready for collection or dispatch. In some cases where we have to deal with extraordinarily fouled/contaminated DPF'S, such as catastrophic engine failure, we cannot reliably predict the cleaning time other than to indicate that it will likely be longer.

8. Is the cleaning process safe for my DPF unit?

Yes, because the non destructive proprietary cleaning process does not effect the DPF'S make-up and is gentle on the walls of the DPF along with the air drying process, the DPF is only subjected to its normal operating flows and pressure.

We do not use high pressure air jets, or extreme temperatures, which may promote cracking of the DPF element.

9. I have a catalytic converter and DPF unit can they both be cleaned?

Yes we can clean both even if they are integrated into a single canister.

10. Can you clean the EGR Valve?

Yes, our process is suitable for cleaning the EGR valve. (Additional fee may be required)

11. The DPF is a single unit that cannot be disassembled, can DPF Deep Clean process it?

Yes, because our process system is more flexible that high pressure air cleaning, we can clean DPF'S of any shape or configuration.

12. If I had a catastrophic engine failure, can my DPF be cleaned?

We would in most cases say yes, even though generally engine oil will be shot into the DOC (Diesel Oxidation Catalyst) unit and the DPF. It will cost more to clean a unit which has been contaminated with oil or excessive fuel, but generally these can be cleaned for a fraction of the cost for a new system. In cases where the engine coolant has also contaminated the filter it may not be possible to clean the unit, in these cases the unit is frequently damaged beyond repair.

13. My DPF is completely blocked, can it be cleaned?

It will depend on why the DPF is blocked. A severely filled DPF can often seem completely blocked but will clean normally. A DPF which has experienced serious overheating during regeneration can sometimes actually melt inside, which will destroy the DPS'S structure and cannot be repaired.

14. Is there any water left in the DPF after cleaning?

By testing using a humidity meter we do not expect the DPF to be holding water, if there is a small amount of moisture left after cleaning (clean water) it will evaporate quickly once installed. In some cases, where a rush job is required, slightly more moisture may be left which will also blow off/evaporate in use.

15. How do I know if my DPF unit is damaged (Cracked)?

Cracks in the DPF substrate are detected by two means.

A crack will manifest itself as a low back pressure and

May also show soot in the engine exhaust.

16. How do DPF Deep Clean verify the results of cleaning?

Cleanliness and performance of a DPF Filter after cleaning can be verified by our 9 stage processing being recorded and we will issue all test results as a method of recording onto the vehicle/plant maintenance records, detailing the contaminant levels pre and post cleaning.

17. Can DPF Deep Clean remove and refit the filter unit?

We aren't currently able to provide that service. The DPF needs to be removed and replaced by your mechanic.