CARMAKERS have two methods for dealing with the gases that belch from exhaust pipes. One is to reduce emissions of carbon dioxide, nitrogen oxide (NOx) and other nasties by spending heavily to develop cleaner engines. Another is to devise methods to game emissions-testing systems but keep polluting the atmosphere on the road.

Volkswagen and Mitsubishi opted for the second method, using means illegal in some countries. But many other carmarkers bend the rules: after VW’s deception came to light, for instance, independent tests showed that across the board, official NOx figures in Europe were a far cry from expectations. This persistent gap between test results and what can be achieved in practice undercuts limits imposed by governments to curtail greenhouse gases and air pollution. Enforcement regimes are in need of repair.

Mitsubishi, Japan’s sixth-largest carmaker, saw its shares plunge after it admitted on April 20th that it had improperly conducted fuel-economy tests. Its potential punishment in court for 25 years of rule-breaking is, as yet, unclear. The vast complexity of Japan’s emissions regime clouds proceedings.

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