In the USA, the EPA (environment protection agency) reckon that some VW diesel models contain software that turns off emissions controls when driving normally and turns them on when the car is undergoing an emissions test.take your pick from the following links:
Volkswagen Accused Of Cheating Emissions RulesVolkswagen under investigation over illegal software that masks pollution | Business | The Guardian
Volkswagen could face $18 billion penalties from EPA| Reuters
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