“A sophisticated software algorithm on certain Volkswagen vehicles detects when the car is undergoing official emissions testing, and turns full emissions controls on only during the test,” the EPA said in a statement. “The effectiveness of these vehicles’ pollution emissions control devices is greatly reduced during all normal driving situations. This results in cars that meet emissions standards in the laboratory or testing station, but during normal operation, emit nitrogen oxides, or NOx, at up to 40 times the standard.”
http://www.theguardian.com/business/2015/sep/18/epa-california-investigate-volkswagen-clean-air-violations?CMP=Share_AndroidApp_Slack