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What is a DPF and how does it work?

DPF is an abbreviation of Diesel Particulate Filter, sometimes referred to as an FAP – Filtre à parlicules on French vehicles. The DPF or FAP forms part of the exhaust system on some diesel vehicles that traps soot and particulate matter in order to prevent it being omitted from the vehicle exhaust into the atmosphere.

The DPF was fitted to a percentage to of vehicles from around 2002 and became pretty much standard equipment from 2008 onwards.

Over time the soot load of the filter accumulates and it is then that regeneration is required. The soot load is calculated by the engine ECU by monitoring pre DPF and post DPF exhaust pressure by means of a pressure differential sensor. When the pre DPF pressure is a predetermined amount higher than the post pressure, regeneration should take place.

The upside to the DPF is cleaner emissions. The downside is reduced engine power and higher fuel consumption.


There are several different methods of regeneration. Extra heat is required in order to burn off the soot and particulate matter.

Passive regeneration is a natural process that normally takes place under certain conditions like on the motorway when the engine is maintaining speed, under reasonable load and at full working temperature. This helps keep the soot level low but generally isn’t enough. This is why other methods of regeneration are required as well.

  • The most recent and popular method is by the ECU triggering the fuel injectors during the exhaust stroke of the engine in order to raise the exhaust temperature.

  • Secondly by the means of a device that injects a small amount of diesel directly into the exhaust in order to raise the exhaust temperature.

  • Thirdly by the means of an additive tank and pump that injects a small amount of a special fluid into the fuel tank every time the driver refuels. This fluid serves as a catalyst that helps burn off the particulate matter. This was a popular system used by Peugeot and Citroen.

DPF Faults

DPF faults are becoming increasingly more common. There are several common causes -

  • Restricted flow due to excessive soot accumulation. This can occur when the vehicle is only covering short journeys so the criteria required for regeneration isn’t met. It can also occur due to engine running faults like a sticking EGR valve or faulty injectors.

  • Excessive ash accumulation. Ash can collect in the DPF when the incorrect engine oil is used. All DPF equipped vehicles must have specific low-ash oil used in them. The ash causes often permanent irreparable damage to the DPF.

  • Inability to regenerate due to a system fault for example- a faulty pressure differential sensor or a split hose going to the sensor.

What are my options?

At Blue Flash Tuning we have the equipment and expertise to diagnose DPF faults on most vehicles. We will always advise the customer of all of the options available allowing the customer to choose the most cost-effective solution.

The DPF is quite an expensive part to replace but fortunately there are several options to consider before replacement -

  • Correct diagnosis of the fault is vital in order not to waste time and potentially vast sums of money.

  • On some vehicles ‘Forced regeneration’ can be conducted. This is by the means of a diagnostic tool telling the ECU to regenerate the DPF.

  • There are now specialist companies that will open the DPF and either clean it or replace the sub-straight.

  • Until a recent change in the law, another popular option was to carry out DPF deletion or removal.
    We DO NOT offer this as an option and urge customers NOT to do it as DPF removal is now an offence under ‘The road vehicles construction and use regulations’.

Please Contact Us with any questions you may have. We are happy to advise and assist where possible.