Instead it was thought to be a weakness in the testIt is amazing that the Volkswagen and diesel emissions scandal was not discovered earlier. As we approached the 2010 legal compliance date, concentrations from traffic went up, not down, and diesel cars were shown to be much more polluting than the official tests led us to believe. However, differences between real-world and official test emissions are not confined to VW. These include reducing the pollution control after around 20 minutes (the official test duration) or when driving below the 20oC test temperature. We will have to wait until September 2019 before all new diesel cars are required to pass a real-world test.

Source: The Guardian March 12, 2017 21:22 UTC