Diesel particulate filters - guidance

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.

Regeneration of DPFs

DPFs need to be emptied of trapped particulate matter regularly. This is carried out automatically by the vehicle during 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

The inspection of the exhaust system carried out during the MOT test includes 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.

It is an offence under the Construction and Use regulations (Northern Ireland) 1999 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.