Almost 1.3 million vehicles failed MOT tests in 2020 because of faults relating to exhaust emissions.
Diesel cars saw the greatest surge in failures due to emissions, with a huge rise of 240% compared to just 37% for petrol cars.
The Freedom of Information (FOI) request to the DVSA by BookMyGarage found that more cars failed on emissions between 2019 and 2020 than any other year before it.
Overall failures in 2020 were up by more than half compared to 2016/17 levels – the final year before the new regulations were introduced.
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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 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,” said Jessica Potts, Head of Marketing at BookMyGarage.com.
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.
A diesel particulate filter is designed to trap soot particles from exhaust emissions, which are toxic to humans.
The 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 failures were classed as dangerous, meaning the car should not be driven until the fault is rectified.
Both petrol and diesel vehicles can fail on a number of items relating to exhaust emissions under 'Section 8.2. Exhaust emissions' of the MOT manual. 
For both fuel types, various inspections are made to the exhaust system, to check for leaks, excessive emissions levels, excessive smoke and the function of emissions control systems.
Generally speaking, only diesel cars have particulate filters as most petrol cars have only been equipped with these filters in the last couple of years.
Jessica added: “Since the Volkswagen ‘dieselgate’ scandal in 2015, diesel cars have earned a bad reputation for producing harmful exhaust emissions.” According to the SMMT 1, 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 2.
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 licenced 3 failing annually due to emissions, compared to 3.3% for diesels.
Notes:
1- Source: https://www.smmt.co.uk/vehicle-data/car-registrations/
2- Source: https://www.gov.uk/government/statistical-data-sets/veh02-licensed-cars#licensed-vehicles, Table VEH0253
3- Source: https://www.gov.uk/government/statistical-data-sets/veh02-licensed-cars#licensed-vehicles, Table VEH0203
