The Environmental Protection Agency issued the German automaker a notice of violation and accused the company of using software known as a “defeat device” in 4-cylinder Volkswagen and Audi vehicles from model years 2009-15. The device is programmed to detect when the car is undergoing official emissions testing. Only during such tests are the cars’ full emissions control systems turned on. During normal driving situations, the controls are turned off, allowing the cars to spew as much as 40 times as much pollution as the legal standard required under the Clean Air Act, the E.P.A. said.
The rumor is the defeat device improves mileage and performance.
Edited for spelling of the brand name! Thanks to vdcdriver, my bad was originally posted as Volkswagon