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.
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.