‘The MOT test changed on 20 May 2018, with new defect types, stricter rules for diesel car emissions, and some vehicles over 40 years old becoming exempt.’ Gov.co.uk
If this defect is issued it means that a vehicle poses an imminent risk to drivers, other road users or the environment. Having this fault on a vehicle makes it undrivable. A vehicle with a dangerous fault will also need to be repaired and retested before getting back on the road.
New changes are coming for diesel vehicles too with stricter rules being put in place. One of the new regulations is if any smoke is seen emitting from the vehicle it will automatically receive a major fault. The diesel particulate filter (DPF) will be checked during the test and if it’s been tampered with, it will result in an automatic fail. Many drivers remove the DPF from time to time to clean it and it’s not illegal to do so. However, it’s illegal to drive without a DPF.