Using portable measuring equipment with hoses attached to vehicle exhaust pipes, researchers drove the Jetta and BMW through Los Angeles and took the Passat to Seattle and back. They also worked with the California Air Resources Board’s laboratory in El Monte, which tested the cars on a dynamometer, a device that measures engine performance.
When the Volkswagen cars were in the lab, they met the Clean Air Act standards. In the real world, they were belching out nitrogen oxides at much higher levels than allowed.