Diesel (DPF)


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.

Why do we have DPF's

With the introduction of Euro 5 emission standards that were brought into force in 2009 new cars were fitted with a DPF (Diesel Particulate Filter) with the aim to deliver an 80% reduction in diesel particulates (soot). In fact, many vehicles registered before 2009 will have one fitted too in anticipation of the change of standards, unfortunately though the technology is not free from problems.

Diesel Particulate Filters

What is a Diesel Particulate filter?

A diesel particulate filter (DPF) is a device fitted to a diesel vehicle which filters particulate matter (PM) from exhaust gases. It does this by trapping solid particles while letting gaseous components escape. This type of filter has been in use for over 20 years, and many variants exist. These filters enable reductions in emissions which help meet European emission standards, improving air quality and thereby health standards.


DPFs need to be emptied of trapped particulate matter regularly. This is done by a process called regeneration, which involves burning the soot to gas at a very high temperature, leaving behind only a very small residue. Regeneration, If not carried out properly, can lead to a build up of soot which can affect performance and ultimately lead to expensive repair costs. This has led to some diesel vehicle owners removing their DPFS. However, DPF removal has both legal and social implications.

Legal Requirements and the MoT test

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.

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