If you are on the internet researching DPF it is more than likely you are unfortunately experiencing some DPF problems, or have suddenly gained an interest in the world of DPF’s! Either way I hope the information here will help.


Not all DPF’s need cleaning, read on…

If you are showing just a DPF light on the dash this may be advising you of a pending DPF issue, which can possibly be averted by taking the car for a drive. Not all manufacturers unfortunately give you this early warning system.

The car hand book should give you the procedure, if not check the link below ‘How to clean the DPF yourself’, this will give you the general procedure.

If by following the instructions  to clean the DPF yourself hasn’t worked – or you have more than just the DPF light, you may have the engine light, coil light or traction control. You will need further assistance which we are able to help you with.

It may be that we can carry out what is known as a Forced Regeneration, to resolve the issue and clear the DPF. We would also need to look into the reason why the DPF has been unable to maintain itself. A blocked DPF should be looked at as a symptom to another problem, this can be engine fault, wrong drive cycle or perhaps emissions related. Just resolving the DPF and not discovering the reason you may well find yourself with a blocked DPF again in no time at all. The most common issue we see is incorrect drive cycle, the section on ‘understanding DPF’s’ may help.

DPF Cleaning (with 12 month Guarantee!)*

Providing the DPF is complete and undamaged internally we can clean it. In the years we have been working with DPF problems, this is the first process that we can say we will successfully clear all ash accumulation and oil contamination. So on high mileage vehicles or vehicles that had perhaps had a turbo failure putting large amounts of oil down through the exhaust, previously there has been no other option than to offer a replacement DPF.

The process works by initially back flushing the DPF with a heated water based solution, flushing is probably an understatement given the force the solution is forced through. The DPF is then turned and flushed in the opposite direction for another round of cleaning. Finally, it is turned once more for a final clean. The DPF is then dried and a pressure reading is taken to compare with the readings taken prior to the start of the process. The readings taken will show flow rate and back pressure within the DPF.

Certain manufacturers have built within their engine control units (ECU) an end of life for the DPF, this can be triggered by either mileage or the DPF reaching a set state. Again providing it is intact internally this can be rejuvenated because of the ability to clear ash. Ash cannot be cleared with any other car cleaning solution, as it has to leave the way it came in.

*Every clean comes with a 12 month guarantee, the labour to remove and refit is not included within the guarantee.

The Diagnostic Process

So if you have a blocked DPF, there is a reason why the filter has been unable to maintain itself. Not doing the diagnostics will more than likely result in the DPF blocking again.

One of the most common causes for a blocked DPF is incorrect driving cycle, if you are not allowing the vehicle the opportunity to regenerate then it will take no time at all to block the DPF again. One of the most common and simplest to resolve problems that will prevent a DPF from regenerating is a lack of fuel! As a general rule if the car has less than quarter of a tank it will prevent the regeneration from starting.

So when the DPF light comes on it is best not to ignore it, as previously mentioned there may be a missed opportunity for you to resolve it yourself if you don’t act promptly. This can vary from manufacturer to manufacturer so it is always worth checking your handbook. If you have the DPF light on its own, this can sometimes indicate that it needs a drive to regenerate. It can sometimes be as simple as taking the car on a motorway drive as described above and wait for the light to go out. Although, if the soot level has gone beyond manageable levels it will not regenerate without diagnostic help, and a visit to a specialist, such as Herts Auto Tek, will be required. (see ‘How to clean the DPF yourself’ below)

Click Below for more information

'How to clean the DPF yourself'

How to clean the DPF yourself

If you want to try and regenerate the DPF yourself first check the handbook for the specific instructions from the manufacturer. Believe it or not, not all manufacturers will have information relating to the DPF in their handbooks!

  • Make sure you have the all important quarter of a tank of fuel
  • The only light on should be the DPF light, engine management light and coil light are on this process will not work.
  • The car needs to be brought up to running temperature before the process will start.
  • Select a road where you can drive without stopping for 15 minutes or 15 miles
  • Use an appropriate gear to drive the car at 2000 rpm and above 45 mph
  • Once the process has ended successfully the light will go out.
'Understanding DPF's'

Understanding DPF’s

This is not a definitive answer to all things DPF but I hope a useful overview on how they work, what can affect their function/process and how you can prolong their life span. It’s worth a read as it is rare that the DPF problem started with the DPF, more often than not the DPF is a symptom relating to another problem.

The DPF seems to me to be a bit of a Band-Aid applied to the diesel emission problem. Car manufacturers when creating a vehicle seem to be working from the tail pipe backwards as it is all about the tail pipe emissions. So the DPF’s function is to store the soot until it’s full, then during the correct driving cycle burn off the soot which then leaves via the tail pipe.

The criteria required for a regeneration means it is unlikely to ever regenerate in town meaning the inner city air quality is improved. Once the car is on the open road out of town the regeneration is then most likely to occur, so it won’t be long until we have asthmatic cows!

How a DPF works: So the DPF catches the soot from the engine and holds it until a certain amount of soot has accumulated, the engine control unit (ECU) will then wait until a type of driving cycle is met for it to trigger the regeneration process(Don’t forget also the requirement of a quarter of a tank of fuel).

Typically the driving cycle will be steady driving above 45 mph, accelerating, braking and stopping will cause it to abandon the regeneration process. So as you can imagine to keep a car running at a steady speed uninterrupted in the normal daily usage of the vehicle can be a challenge within itself.

During the regeneration process additional fuel is delivered into the engine to generate heat within the DPF, soot typically burns at 550 degrees celsius and above so this temperature needs to be maintained throughout the process. This causes the soot to burn off and each regeneration will leave a very small amount ash behind which remains trapped in the DPF.

After many regenerations the (ECU)will estimate the amount of ash that has accumulated within the DPF and flag a code relating to ‘Ash Accumulation’. This has been to now regarded as end of the DPF life.

'Recent Examples of misdiagnosis'

‘Recent Examples of misdiagnosis’

We have come across a real mixed bag of faults which have all come to us under the banner of a ‘DPF fault’ below are a few recent examples.

A BMW X3 diagnosed by a main dealer as needing the cylinder head de-coking and DPF replacement, turned out to be a turbo fault and the DPF was fine. After paying £500 for just a diagnosis, and being given an estimation of £4000 he was pleased to have the car back on the road for less than half the main dealers estimated cost!

A Skoda Fabia kept being force regenerated by main dealer who overlooked low fuel pressure code, now fuel pressure problem resolved by changing the fuel filter,  the car regenerates all by itself.

A Toyota Avensis before coming to us had already had a turbo changed and 2 x DPF’s fitted. Turned out to be carbon blocking one of the differential pipes causing a misreading pressure sensor. The misdiagnosis had cost the customer over £3000 before he came to us! Facebook link

A Toyota Avensis that had been diagnosed as needing a new DPF, when we attempted to clean the real problem came to light of a fault EGR valve. With a temporary fix to blank the EGR we successfully cleaned the DPF and replaced the EGR. Facebook Link

'DPF Removal'

The Law has changed!

The government has introduced a change to the MOT as from 16th February 2014. If a diesel particulate filter (DPF) has been removed from a car that had one fitted as standard it will fail the MOT. This is a visual check only for the presence of the DPF and catalytic converter.

Notice issued to MOT stations

On 16th February 2014 Section 7.1 of the inspection manual will be amended to include a check for the presence of catalysts and particulate filters on diesel powered vehicles.  Any vehicle where a catalytic converter or particulate filter is missing where one was fitted as standard will fail the test.

If you are planning to export or use the vehicle for ‘off road use only’ in other words do not require to have the vehicle MOT tested then you can still consider having the DPF removed. We would recommend though you carry out your own research on this.

(Updated 02/03/18)


Draft changes to the MOT for May 2018 are now available online to read, with reference to DPF’s there is a proposed inspection to see if there are any welding marks on the DPF case to suggest it has been tampered with. If spotted by the MOT examiner an explanation and supporting documentation will need to be provided for an MOT pass.

“Diesel particulate filters (DPF) should be checked for evidence that the DPF has been removed or otherwise tampered with. Where a DPF canister has clearly been cut open and re-welded, it should be rejected unless evidence can be provided that the canister was cut open for legitimate reasons, such as filter cleaning.”

You can read the whole draft here:-


Next Steps

Opening Hours

Mondays to Fridays
8 am – 5 pm

Late night Thursday by appointment

Herts Auto Tek
Unit A4 Front
Nup End Bus. Centre
Herts. SG3 6QJ

Tel: 01438 829000