How Volkswagen programmed its cars to skirt the law

Today, the Environmental Protection Agency forced German automaker Volkswagen to recall and fix nearly half a million VW and Audi cars. VW also faces billions in fines. From the Associated Press:

The cars, all built in the last seven years, include a device programmed to detect when they are undergoing official emissions testing, the EPA said, adding that the cars only turn on full emissions control systems during that testing. The controls are turned off during normal driving situations, the EPA said. The EPA called the company’s use of the device illegal and a threat to public health.

The software, known as a defeat device, was installed in 4-cylinder VW and Audi vehicles manufactured between 2009 and 2015. It’s meant to “conceal the cars’ emissions of the pollutant nitrogen oxide, which contributes to the creation of ozone and smog,” according to the New York Times. These compounds can lead to respiratory health problems.