Volkswagen has been cheating the emissions testing system to make its diesel cars seem less polluting than they really are. But how did it do this? New vehicles have to meet ever stricter emissions standards for a range of pollutants, but the testing is done in a lab rather than on the road. Volkswagen programmed its onboard software to detect when cars with its TDI diesel engine were undergoing an emissions test, using information from the steering, brakes and accelerator. It then tweaked the engine settings to minimise levels of nitrogen oxides (NO_x). On the open road, NO_x emissions are up to 35 times higher. Up to 11 million vehicles made from 2009 onwards may be affected. It is not yet clear exactly what this software does, but we can make an educated guess. There are two main ways to reduce NO_x emissions: minimise their production or remove them from exhaust fumes.
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