The diesel cars were programmed to sense when emissions were being tested and to turn on equipment that reduced emissions, according to United States officials. At other times, the cars had better fuel economy and performance, but produced as much as 40 times the allowed amount of nitrogen oxide, a pollutant that can contribute to respiratory problems including asthma, bronchitis and emphysema.

What cars are affected by this?

The cars are 2009 to 2015 TDI Volkswagen Golf, Jetta, Beetle and Audi A3s, and the 2014 to 2015 Passat. All are powered by the company’s 2.0-liter turbodiesel four-cylinder engines. There are other diesels engines in the greater VW stable, but those aren’t affected here as far as we know.