Currently reading: Legal fears over aftermarket exhausts following DVSA probe
"Grey area" over emissions limits causes confusion for garages and threatens further prosecutions

A workshop being found guilty of carrying out illegal alterations to a car has exposed a grey area in the law that puts anyone who modifies a car at risk of prosecution, another tuning specialist has warned.

In November last year, Wakefield-based AET Motorsport was ordered by a court to pay £7234 in fines and costs after the Driver and Vehicle Standards Agency found it had made alterations to a car to remove its catalytic converter and modified its engine control unit to increase noise levels, making it illegal to drive on a public road.

Magistrates told AET Motorsport that the case was rightly brought by the DVSA and that business owners were obliged to know the law.

However, James Wills, director of Newton Abbot-based Auto Dynamix, has claimed that while the court’s judgement was right, the case has exposed contradictions in the law concerning exhaust emissions that he believes need clearing up.

“A de-cat exhaust system on a road-going car is illegal and will cause a car to fail its MOT,” he said. “So to achieve near enough the same performance results, drivers are having sport catalytic converter systems fitted to their cars instead. These are sufficient to get the car through the MOT, but I doubt they satisfy the emissions limits set at the car’s type approval stage.”

To enable better gas flow, a sport exhaust may contain less than half the catalyst material of the car’s standard system or, in the case of a car with multiple catalytic converters, considerably less than that. However, so long as it’s an EC-marked system and the car’s emissions are within limits at the required low engine speeds when there’s also no load on the engine, a vehicle fitted with an approved sport cat (ideally supported by modified engine management software) will satisfy the MOT regulations.

Sports catalytic converters

The problem arises when, away from the MOT test centre, with the engine running at higher speeds and with a turbocharger boosting, it is likely to produce emissions that exceed the type approval limits specified for the vehicle when it was certified with its standard cat exhaust system.

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“For this reason, I’m concerned that were an inspector to properly test the emissions of a car fitted with a sport cat across its operating range, they would find it illegal,” Wills told Autocar. “This contradiction between MOT and vehicle type approval regulations needs to be sorted out before someone else is prosecuted.”

Autocar asked the DVSA to clarify the position concerning the type approval status of a car fitted with a sport cat. A spokesman said simply that it’s an offence to use on a road a vehicle that has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet, and therefore a vehicle found not to comply with type approval limits could be illegal.

He added, however, that since not every vehicle is type approved, it’s “very rare” that, for example, the emissions of a vehicle being used on the road would be measured [by the DVSA] in the same way as for vehicle type approval.

The spokesman continued: “Manufacturers don’t type approve every vehicle; they submit a range of vehicles to the VCA [Vehicle Certification Agency] to test and approve. The MOT then ensures that all in-service vehicles from year three meet the required standard for that manufacturer that broadly aligns to type approval standards. Alongside the VCA, the DVSA carries out market surveillance to ensure that vehicles continue to meet type approval standards.”

Mercedes in WLTP exhaust emissions testing

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Wills was unimpressed by these comments, saying: “On the one hand the DVSA is saying a vehicle must meet its type approval limits, but on the other it’s saying that it rarely checks them because the tests are different and in any case not every vehicle is type approved. For this reason, drivers need to be aware that although their car might pass the MOT test, they could still be prosecuted by the DVSA for breaching emissions regulations.”

Manufacturers of sport cats that Autocar has spoken to have agreed that their position is unclear.

A spokesman for one said: “Sport cats are a grey area. Our product complies with the legal limits but changes those limits higher up the engine’s rev range.

“Given the recent news concerning AET Motorsport, we’re revising the wording of our sales literature.”

A spokesman for the Performance Automotive Aftermarket Association, an organisation that represents major performance aftermarket manufacturers, distributors and retailers in the UK, said that it is working with the DVSA to clarify the position for its members.

Meanwhile, to help drivers identify responsible suppliers and installers of aftermarket products, it has introduced the Smart mark. Standing for ‘safe modification and responsible tuning’, Smart will be applied to products the organisation deems to be safe, road-legal and fit for purpose.

“We’re trying to give people an informed choice,” said a spokesman.

What are the differences between the vehicle type approval and MOT tests?

MOT station

The type approval test is a laboratory test that measures how many grams of CO2 a vehicle emits per kilometre. It’s conducted by the Vehicle Certification Agency or another EU-approved body. It measures the specific outputs from the exhaust in relation to hydrocarbons, CO, NOx and other gases and particulates. Tests are performed across a particular set of driving conditions so that the results can be compared between vehicles.

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The MOT test is performed on an in-service vehicle in a garage and measures the amount of CO2 as parts per million within the vehicle emissions, quantified as a percentage of the total amount of emissions. The basic test is carried out at a fast idle (2500-3000rpm) and at a normal idle (450-1500rpm). If the vehicle has a catalytic converter but fails the basic test, it is subjected to the more thorough extended emissions test.

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Add a comment…
turbinecol 9 February 2024

They don't measure C02, just CO on the MOT. As C02 is of course a harmless gas. 

Bob Cholmondeley 11 February 2024

CO2 is not harmless but, ignorance is bliss, as they say. 


Keep burying your head in the sand...

scrap 9 February 2024

Folk removing cats from exhausts are scumbags.

The law exists to limit poisonous fumes being emitted into the air we all breathe. Have a word with yourselves.