A record number of vehicles – almost 1.3 million – failed their MOT last year, because of 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.

Overall failures last year were up by more than 70% compared to 2017/18 levels – the final year before the new regulations were introduced.

Diesel vehicles have seen the greatest surge in failures due to emissions, with a rise of 240% compared to just 37% for petrol vehicles, says BookMyGarage.com, which tabled the FOI.

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.

Jessica Potts, head of marketing at BookMyGarage.com, 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).

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.

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

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.

“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.”

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.

Financial Year	Diesel	Petrol	Total
2015-2016	118,302	748,465	868,115
2016-2017	122,838	690,247	814,684
2017-2018	123,596	620,247	745,308
2018-2019*	397,991	910,620	1,311,841
2019-2020	420,537	849,740	1,273,771

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, click here.