Almost 1.3 million vehicles failed their MOT last year due to faults relating to exhaust emissions, new data suggests.

A Freedom of Information (FOI) request to the DVSA found more cars have failed on emissions in the past two years than any other before it., which tabled the FOI, said diesel vehicles saw the greatest surge in failures due to emissions, with a rise of 240% compared to just 37% for petrol vehicles.

Failures last year were up by more than 70% compared to 2017/18 levels (the final year before the new regulations were introduced).

May 2018 saw the Government introduce tougher MOT regulations to clamp down on vehicles producing excessive emissions, which led to a significant rise in failures.

Jessica Potts, head of marketing at, said: “The regulations have mostly impacted diesel cars, causing more than triple the number to fail, compared to petrol car failures which have only increased by a third”.

The large increase in diesel failures was caused by a change to rules for cars equipped with a diesel particulate filter (DPF). These became standard on all diesel cars in 2009 to comply with Euro 5 emissions standards. Its purpose is to trap soot particles from exhaust emissions which are toxic to humans.

Any car equipped with a DPF will fail an MOT if there is either evidence it has been tampered with or if smoke of any colour comes from the exhaust.

DVSA also introduced new fault categories, with ‘Major’ or ‘Dangerous’ faults resulting in a failed test.

In 2020, almost all petrol emissions failures were classed as ‘Major’ and 5% of all diesel emissions failures were classed as ‘Dangerous’, meaning the car should not be driven until the fault is rectified.

Potts explained: “Since the Volkswagen ‘dieselgate’ scandal in 2015, diesel cars have earned a bad reputation for producing harmful exhaust emissions.

“That’s not to say all diesels are bad,” continued Potts. “The latest diesel cars are equipped with emissions control systems such as particulate filters and selective catalytic reduction (AdBlue) to reduce or eliminate harmful emissions.

“What this data tells us though, is that an increasing number of relatively modern diesels are struggling to pass the MOT test as their emissions control systems face tougher scrutiny. It’s important these systems function correctly to protect the environment but putting them right can also cost owners thousands of pounds”.

According to the SMMT, the market share of diesel cars accounted for just 16% of new car sales last year. In 2015, about 50% of new cars sold were diesel.

Although diesels have seen a much larger failure rate increase in recent years, petrol cars are actually still more likely to fail, with 4.5% of the total number licenced failing annually due to emissions, compared to 3.3% for diesels.