Underlying U.S. and EU emission standards
The Volkswagen and Audi cars identified as violators had been certified to meet either the US EPA Tier 2 / Bin 5 emissions standard or the California LEV-II ULEV standard. Either standard requires that nitrogen oxide emissions not exceed 0.043 grams per kilometre (0.07 g/mi) for engines at full useful life which is defined as either 190,000 kilometres (120,000 mi) or 240,000 kilometres (150,000 mi) depending on the vehicle and optional certification choices.
This standard for nitrogen oxide emissions is among the most stringent in the world. For comparison, the contemporary European standards known as Euro 5 (2008 "EU5 compliant", 2009[5]–2014 models) and Euro 6 (2015 models) only limit nitrogen oxide emissions to 0.18 grams per kilometre (0.29 g/mi) and 0.08 grams per kilometre (0.13 g/mi) respectively. Defeat devices are forbidden in the EU. The use of a defeat device is subject to a penalty.