In the UK it is an offence to use a vehicle on the road that no longer complies with the emissions standards it was designed to meet.
Changing any part of the exhaust or emissions system with a part other than the vehicle manufacturer’s original part that may change the emissions could be an offence. Currently emissions testing outside the London Emissions Zone (LEZ) is limited to the standard MOT test.
From February 2014 the inspection of the exhaust system carried out during the MOT will include a visual check for the presence of a DPF canister, if fitted as standard equipment.
In Ireland the NCT will soon start checking Diesel Particulate Filters.