Some manfs were cheating - they increased tire pressure to levels that would be dangerous on roads, for instance.<p>Some manfs were building to the type approval test, but not for real world driving.  A car needs to accelerate from 0 kph to 100 kph over a certain time.  Normal road drivers would accelerate faster than this test, and acceleration is when the vehicles emit a lot of stuff.  The tests are run on engines at their normal operating temperature.  So cold engines are allowed to emit more stuff to protect the engine.  Many engines in real world use would be at this cold temperature.<p>The US has particularly stringent testing, and it might not be possible to meet those tests.  They should probably have done other stuff to reduce diesel vehicles - taxes and duties on the vehicles and the fuel, with exemptions for commercial operators.<p>It&#x27;s clear now that manfs build to the test, and that type approval testing needs to include better real world driving testing.