DPF (Diesel Particulate Filter) is a device designed to remove diesel particles or soot from the exhaust gas of a diesel engine. It basically traps the diesel particles in the filter and at a given condition the ECU will put it into regeneration mode when these particles are burnt and blown out of the exhaust. Unfortunately these filters get blocked by constant urban cycles which may cause running problems. The engine will enter in “limp mode” and the warning MIL will be displayed on the dash panel.





The repair costs can be very expensive, typical examples:


DPF filter £800 to £2300 (depending on vehicle type & based on OEM Filters Only )


Additive refill £150


Additive ECU £180


Pressure sensor £90


Increased Oil & Filter changes due to failed regenerations (low SAPS full syn oil) £70





Regeneration is either passive or active


Passive regeneration


Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don't get this sort of use car manufacturers have 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 ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is a bit stop/start or you take your foot off the accelerator 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 start a complete regeneration and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph


If the regeneration is unsuccessful the extra fuel injected will not burn and will drain into the sump.  Oil quality will deteriorate as a result of this and the level will rise.  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 excess engine oil – often to the point of destruction. This condition effects some cars more than others, some of worse are  Japanese and Korean cars (mazda 6 & 5 are very common)







Before the Latest Government announcement (9/12/13 ) There was the option to have the DPF filter removed. This service can no longer be offered due to current legal issues.


Removal is not a legal option


It is suggested from time to time that the answer to a 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.


Also from Feb 2014 , The DPF filter is Part of the current MOT / VOSA annual test. 


Legal requirements and the MoT test 


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.


A vehicle might still pass the MoT visible smoke emissions test, which is primarily


intended to identify vehicles that are in a very poor state of repair, whilst emitting


illegal and harmful levels of fine exhaust particulate.