In its complaint on Tuesday, the Justice Department noted that the engine controls in the Ram trucks and Jeeps are different from what are described in paperwork the company filed with the E.P.A.
The features of the software, it said, “alone or in combination with one or more of the others, bypass, defeat and/or render inoperative” the vehicles’ emission control system, causing them to emit “emit substantially higher levels” of nitric oxide than allowed.
The E.P.A. made its accusation after Volkswagen’s admission that it had used “defeat device” software to enable its cars to pass emissions tests while spewing far more pollutants than allowed in normal driving.
Volkswagen, of Germany, ended up paying billions of dollars in fines, several of its executives have been investigated or charged with crimes. As part of a settlement with authorities in the United States, Volkswagen is buying back hundreds of thousands of diesel cars from American customers.
United States officials view the Fiat Chrysler matter as less serious, and stopped short of accusing the company of intentionally engineering the software to cheat on emissions tests. Filing the suit is seen as an effort to accelerate settlement negotiations, after the company outlined its proposal last Friday to reset the engine software in the affected vehicles.
The suit was filed by the Justice Department’s environmental enforcement section in United States District Court for the Eastern District of Michigan.
The emissions issue has essentially stopped Fiat Chrysler’s domestic sales of diesel-powered Ram 1500 trucks and Grand Cherokees. In 2015 and 2016, the company sold more than 50,000 diesel Ram 1500 pickups a year in the United States, making it by far the largest-selling diesel-powered light vehicle in the country.
But the company has been unable to sell Ram 1500 and Grand Cherokee diesel vehicles in the United States for the current model year because the E.P.A. has not certified that they meet emissions regulations. It continues to sell larger Ram 2500 and 3500 models, which are equipped with a different diesel engine.
Before any resumption of sales, the E.P.A. would have to test and approve the new engine software that the company has developed, a process that could take weeks or months.
Once the software is certified, Fiat Chrysler could alert owners to the update by a recall or possibly a less formal customer-service campaign. The method will be determined in negotiations with the E.P.A. Either way, customers would be notified by mail and other means to bring their vehicles to a dealer to have the software updated at no charge.
An earlier version of this article overstated the impact of the emissions issue on Fiat Chrysler’s domestic sales of diesel-powered trucks. While the company has been unable to sell Ram 1500 and Grand Cherokee diesel vehicles for the current model year, it continues to sell larger Ram 2500 and 3500 models, which have a larger diesel engine; it is not the case that Fiat Chrysler has been unable to sell diesel vehicles in the United States for the current model year.