Almost 1.3 million vehicles failed MOT tests last year because of faults relating to exhaust emissions.
BookMyGarage.com found that diesel vehicles have seen the greatest surge in failures due to emissions, with a rise of 240% compared to 37% for petrol vehicles.
The Freedom of Information (FOI) request to the DVSA by the MOT and service price comparison site found more cars have failed on emissions in the last two years than any other before it.
Overall failures last year were up by more than 70% compared to 2017/18 levels – the final year before the new regulations were introduced.
In May 2018, the government introduced tougher MOT regulations to clamp down on vehicles producing excessive emissions which led to a significant rise in failures.
The large increase in diesel failures was caused by a change to rules for cars equipped with a diesel particulate filter (DPF). 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 can be seen coming from the exhaust.
DPFs became standard on all diesel cars in 2009 to comply with Euro 5 emissions standards, though a few cars older than this may also be equipped with a DPF. Its purpose is to trap soot particles from exhaust emissions which are toxic to humans.
DVSA also introduced new fault categories, with ‘Major’ or ‘Dangerous’ faults resulting in a failed test.
Almost all petrol emissions failures were classed as ‘Major’ last year. By comparison, around 5% of all diesel emissions failures were classed as ‘Dangerous’, meaning the car should not be driven until the fault is rectified.
Jessica Potts, head of marketing at BookMyGarage.com, said: “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. 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.”
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 licensed failing annually due to emissions, compared to 3.3% for diesels.
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 .
Click here to see an interactive graph comparing annual data for MOT emissions failures and to find out which faults cause a petrol or diesel car to fail the test.