The US government is suing Fiat Chrysler, alleging that some diesel pick-up trucks and Jeep SUVs cheat on emissions tests.
The lawsuit filed by the Justice Department on Tuesday alleges that nearly 104,000 Ram pick-ups and Jeep Grand Cherokees from the 2014 to 2016 model years have software that allows them to emit lower amounts of pollutants during lab tests by the Environmental Protection Agency (EPA) than during normal driving conditions.
The three-litre Fiat Chrysler Automobiles (FCA) diesels emit nitrogen oxide at a much higher rate than allowed under federal laws when on the road, the EPA says in a statement.
The company failed to disclose the software during the process to become certified so the vehicles can be sold, according to the EPA.
The agency called the software a "defeat device" that changes the way the vehicles perform on treadmill tests in a laboratory.
"Each of these vehicles differs materially from the specifications provided to EPA in the certification applications," the statement said.
"Thus the cars are uncertified, in violation of the Clean Air Act."
Fiat Chrysler has maintained that the devices do not detect when the Jeeps and pick-ups are being tested, and they are designed to detect temperature and other conditions that could harm the engines if the emissions controls are turned on.
The company says its use of the software is different from Volkswagen, which used a defeat device to intentionally thwart emissions tests.
The EPA issued a "notice of violation" against FCA, exposing the software in January.
At the time, FCA chief executive Sergio Marchionne denied any wrongdoing and said the agency was blowing the issue out of proportion.
The lawsuit filed in Detroit federal court seeks an order stopping the practice as well as civil penalties, according to the EPA statement.
The agency and the California Air Resources Board are still discussing with FCA ways to make the vehicles comply with federal and California pollution laws.
"The nature and timing of any resolution of this issue are uncertain," the EPA statement said.