Many countries have vehicles with permanently active catalytic converters despite the fact that they lower efficiency and produce more carbon dioxide for the same mechanical power output.

The gases the catalytic converters combust, like carbon monoxide, hydrocarbons and nitrogen oxides, are not dangerous in the concentrations they often appear, and they spontaneously react to become other chemicals after some time in the atmosphere.

Catalytic converters were introduced 35 years ago when environment policies and car configurability were different. I don't question the motives of why the laws were passed then. Today there is a big focus on reducing carbon dioxide emissions, so why are catalytic converters still in permanent use? Catalytic conversion can be avoided with just a software change or configuration change in the engine control unit, that would reduce carbon dioxide emissions by some percent.

A solution could be an adaptive system that configures combustion depending on circumstances, like turning off the oxygen supply to the catalytic converter when the car is moving at higher speed and turning it on in confined spaces like parking places and smoggy cities. It could also easily be turned off and on depending on geographic location or based on data from an carbon monoxide sensor.

Does the law require permanent usage of catalytic converters in cars?

The setting of the engine control unit is the direct cause of increased fuel consumption, since the combustion process in the engine cylinder is providing more air than is optimal in order to provide oxygen to the combustion in the catalytic converter. Back pressure from the catalytic converter also decreases engine efficiency and increases fuel consumption. In some vehicles there is a secondary air injection to provide oxygen to the catalytic converter. This requires energy and increases fuel consumption.

For example in regard to carbon monoxide the catalytic converter can be deactivated when levels are within WHO guidelines at 6 ppm. In average CO levels are 0.1 ppm in the atmosphere.

A common misunderstanding of this question is that people seems to think that I write that the total carbon dioxide produced per unit of fuel will be different with or without catalytic conversion. I have not written that. I write that there will be a diffence per mechanical power produced.

  • You say that catconverters are bad because of inefficiency and excess CO2, and that we must be able to manage better with technology nowadays, but you give no quantitative justification for this, nor even a qualitative one that can be analysed to any depth. For example, you know that at least two-thirds of the energy produced in a modern petrol engine is totally wasted as heat? What difference would a few joules per million for a compressor make, if it reduces toxic UHC and PIC quantities by hundreds of thousands per million particles?
    – user4657
    Commented Jun 26, 2020 at 12:13
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    Also, discussion of whether such laws are or were appropriate, or technical justifications for them, are outside the scope of this site, and some people may be finding that distracting. I would suggest editing your question down to something like "In [country X], does the law require permanent usage of catalytic converters in cars? Or would it be legal to have a car where the catalytic converter can be switched on and off during operation?" Commented Jun 26, 2020 at 12:54
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    @DavidJonsson: Maybe not directly, but you do have an extended explanation as to why you believe optional converters would be a good idea, and that reads more like advocating for the policy rather than simply asking whether it exists. Commented Jun 26, 2020 at 14:51
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    Cut out the pointless rant/posturing/opinion and there's a legal question here.. Also would you mind not going round slagging MVM&R SE off everywhere you post this screed? It's rude, and insulting. Commented Jun 26, 2020 at 16:44
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    I’m voting to close this question because it's a rant. Commented Jul 16, 2020 at 13:53

1 Answer 1


There is no EU law that requires vehicles to have a permanently active catalytic converter installed. There are various "clean air" regulations such as Regulation 715/2007/EC which covers the emissions standards vehicles must meet before they can be approved for sale within the EU. As long as the standards are met, the law is not particularly concerned with how those standards are met (excluding obvious cheating mechanisms, etc.)

Additionally, Directive 2014/45/EU covers vehicle roadworthiness and mandates regular emissions testing as part of it. All vehicles must meet the standards of the test to be considered roadworthy. In the UK, this is performed as part of the annual MOT test.

It appears that catalytic converters are simply one of the easiest ways to meet those standards, and it is probably cheaper/more efficient for manufacturers to install a permanently active catalytic converter (which can be applied across multiple markets) rather than implementing software or alternative solutions which may or may not meet regulations in all the markets the vehicle is sold in.

  • Does EU law or regulations require a car to continue to meet the emissions standards after the car was first sold? Commented Jul 16, 2020 at 16:11
  • Yes. Directive 2014/45/EU covers vehicle roadworthiness and mandates regular emissions testing as part of it. In the UK, this is performed as part of the annual MOT test.
    – Matthew
    Commented Jul 16, 2020 at 16:25
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    Thanks. I think the continuing obligation to meet emissions standards would be useful to readers, and should be edited into the answer. Commented Jul 16, 2020 at 16:28
  • This of course raises the question of whether you could switch off the converter and just switch it back on during your annual test (a la Volkswagen). I know US states have laws that forbid disabling or deactivating a vehicle's emissions systems at any time; does EU have such a law? Commented Jul 16, 2020 at 16:29
  • In the UK, it is a criminal offence to tamper with your car's emissions system. While it may not be a criminal offence across the EU, it would almost certainly be a breach of the relevant EU "clean air" legislation. It's up to each Member State to implement any appropriate criminal offences in their domestic law though.
    – Matthew
    Commented Jul 16, 2020 at 16:39

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