German carmaker Volkswagen has been ordered by US regulators to recall half a million cars because of a device that disguises pollution levels.
The "defeat device" allows cars to pass lab testing even though they actually emit 40 times the emissions standard.
The US Environmental Protection Agency (EPA) has been taking a more aggressive stance on car pollution and violations of the country's Clean Air Act.
The recall could cost the carmaker up to $18bn (£11.5bn).
It affects 2009-2014 Jettas, Beetles, Audi A3s and Golfs and 2014-2015 Passats.
"Using a defeat device in cars to evade clean air standards is illegal and a threat to public health," said Cynthia Giles, assistant administrator for the Office of Enforcement and Compliance Assurance.
The illegal system allowed cars to detect when they were undergoing smog emission test and lowered the rate of pollution. Those emission controls were then turned off during ordinary use.