The carmaker said the move was the âresult of many months of close collaborationâ with the Environmental Protection Agency and the California Air Resources Board. Fiat Chrysler said it thought the updates would help it reach settlements with the E.P.A. and the Justice Department.
The Justice Department has been investigating Fiat Chrysler since the E.P.A. in January accused the carmaker of violating clean-air rules. Fiat Chrysler said this week that it was trying to work out a settlement with the Justice Department, but added that it would fight any suggestion that it had used illegal software to dupe regulators.
In 2015 and 2016, the company sold more than 50,000 diesel Ram pickups in the United States each year, making the model the countryâs largest-selling diesel-powered light vehicle by far, according to HybridCars.com. But the emissions question has kept Fiat Chrysler from selling any diesel Ram or Grand Cherokee vehicles this year because the E.P.A. has not certified them for sale.
The E.P.A. declined to comment about Fiat Chryslerâs announcement.
The company already faces accusations in Europe that its cars produce far more nitrogen oxides in normal driving than during tests. On Wednesday, the European Unionâs executive arm filed a formal complaint against the Italian government for allowing Fiat Chrysler to sell cars designed to evade emissions tests.
And the evidence has piled up that those high emissions levels were part of efforts to evade environmental standards.
Academic researchers on both sides of the Atlantic said separately this week that they had found that Fiat Chryslerâs diesel vehicles had suspiciously high pollution levels and that there was evidence the company had used a so-called defeat device, software intended to allow a vehicle to pass official emissions tests but to pollute more when driven on the highway.
In the latest instance, the American university researcher who exposed Volkswagenâs emissions scandal said in a telephone interview late Thursday that tests of Jeep Grand Cherokees and Ram 1500 had revealed major discrepancies between road and lab emissions.
Dan Carder, director of a renowned vehicle emissions program at West Virginia University, said a diesel Ram pickup tested by the group produced up to 20 times as much harmful nitrogen oxides on the road than under controlled conditions in a lab. A diesel Jeep Grand Cherokee produced up to five times as much.