"Only then did VW admit it had designed and installed a defeat device" that purposely lowered emissions while a vehicle was being inspected, the agency said. During regular driving, emissions would return to as much as 40 times the level of pollutants allowed under clean air rules.
The other sources I've seen specifically said ANY time a vehicle was tested, this did not show up.
Tests are typically done on a dynomometer. One easy solution would be trigger the clean mode when the front (driven) wheels are spinning but the rears are not.
