The Environmental Protection Agency alleged this Friday that automaker Volkswagen intentionally violated clean air regulations during testing. According to an Associated Press report, the company reportedly included software in 500,000 of its diesel vehicles that allowed them to emit fewer pollutants during testing than they would actually emit once on the road.

The agency asserts that the German automaker should repair the vehicles to comply with smog regulations at their own expense. The company also faces billions of dollars in fines for the violations, though no dollar amount has been put forth yet.

The cars were all built over the course of the last seven years, and include Jetta, Beetle, Golf, and Passat models. The Audi A3 was also reported to have this software included during testing periods.

The device planted in the cars detects the start of official emissions testing, and can switches on an emissions control system. The system is shut down once the test is completed, and the cars emit much more smog-inducing pollutants when driving on the road.

The EPA claims that the company’s use of the “defeat device” violated clean air laws and posed an unnecessary risk to public health. While the automaker will be responsible for repairs to the emissions systems, car owners do not need to do anything for the time being. Cars with this software installed are perfectly safe to drive, and can remain on the road until VW comes up with a plan to fix them.

Volkswagen likely disabled the emissions systems to increase engine performance, so they could advertise their cars as both clean and powerful. They will likely face fines of up to $37,500 per vehicle.