The Justice Department is suing Fiat Chrysler, claiming software let some of its vehicles produce more emissions than is legally allowed.
The Environmental Protection Agency says Fiat Chrysler didn't disclose software features in the control system of some of its diesel-fueled Ram pickups and Jeep Grand Cherokees.
The lawsuit also claims the software allowed the cars to emit lower amounts of nitrogen oxide when they were being tested.
Over 100,000 of these vehicles were sold between 2013 and 2016. The EPA alleges that the sales violate the Clean Air Act.
Fiat Chrysler says it wasn't trying to cheat on emissions tests, and the lawsuit doesn't go as far as accusing the company of deliberately trying to.
Some see the Fiat Chrysler lawsuit as a way to speed up a settlement. Last week the company offered a proposal on how it could reset the engine software.