MOT tests on diesel vehicles to be tightened

Changes to MOT tests which come into effect from next month (20 May) will see vehicles checked more rigorously to ensure that measures to limit diesel emissions are in place.

The changes will affect the way that cars, vans, and other light passenger vehicles are tested, and see defects categorised as either ‘dangerous’, ‘major’ or ‘minor’ depending on the type of issue found.

MOT testers have been told to fail a vehicle if they see smoke of any colour emitted from a vehicle’s exhaust

This will include stricter limits for emissions from diesel cars with a diesel particulate filter (DPF).

A diesel particulate filter (DPF) captures and stores exhaust soot in order to reduce emissions from diesel cars, and is legally required to prevent the emission of high levels of particulates.

The filters have a finite capacity, which means that trapped soot has to be emptied or ‘burned off’ regularly to regenerate the DPF.


Testers will be required to test vehicles to the manufacturer’s plate value – the emission limit specified by the car maker — or using new default limits set by the Driver and Vehicle Standards Agency (DVSA).

Vehicles will be given a ‘major’ fault — meaning that the vehicle will fail its test — if the MOT tester is able to see smoke ‘of any colour’ coming from the vehicle’s exhaust. Cars will also fail the MOT if the tester can find evidence that the DPF has been tampered with.

In recent months, concerns have been raised that many drivers are illegally driving vehicles which have had DPFs tampered with or removed (see story).

Under the Road Vehicles (Construction and Use) Regulations (Regulation 61a(3)) using a vehicle which has had its DPF removed is an offence and can incur a fine of up to £1,000 for a car or £2,500 for a light goods vehicle.

Rules were tightened in 2014 to require garages to perform a visual inspection of filters as part of the MOT process. This means that a vehicle will automatically fail its MOT test if the filter had been fitted as standard but is found to be no longer present.

The Driver & Vehicle Standards Agency (DVSA) has claimed that as many as 1,800 vehicle users have been caught operating without a DPF since the MOT testing requirements have changed.

Related Links
DVSA – Diesel vehicle emissions limits


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