Important changes to MOT law covering diesel particulate filters (DPFs) are about to come into force.
Prior to a change in the legislation, introduced in February, cars were only tested on emissions levels and not whether their DPF was attached.
Now any car that should have a DPF but is found to have it missing will instantly fail its MOT test.
Rather than explaining why these important filters become clogged, some garages have been removing the DPF.
This practice has always been illegal but some firms are continuing to advertise the service as though it is a legitimate long-term solution.
This means thousands of owners are now facing the cost of buying a replacement DPF, which can cost up to £1200 to refit.
DPFs often become clogged with soot if a vehicle is frequently used to make short journeys because it does not get hot enough to burn off the particulates that build up over time, unlike motorway journeys.
One of the best ways in which to minimise the problem, and the chances of an MOT failure, is to give the vehicle a good run on a regular basis.