Fiat Chrysler Automobiles' proposed fix for Ram and Jeep vehicles equipped with the company's 3.0-liter diesel V-6 is not enough to satisfy the U.S. Department of Justice as the department has just filed a civil complaint against FCA in regards to the diesel engines, which have been fitted to more than 100,000 vehicles.

The lawsuit was filed on Tuesday in a U.S. District Court in Detroit, Michigan. FCA is accused of using eight undisclosed auxiliary emission control devices in Jeep Grand Cherokees and Ram 1500 pickups that alter emission control systems under various driving conditions.

The control devices, which are legal but must be disclosed and their purpose explained, are said to emit higher levels of nitrogen oxides during real-world driving conditions than during U.S. emission tests. When asked for proof the devices did not have the intention to cheat emission tests, FCA failed to fully explain itself, according to the EPA.

FCA's use of the auxiliary control devices is in violation of the Clean Air Act, according to the lawsuit. The filings also state that the Jeep and Ram vehicles differ from “the specifications provided to EPA in the certification applications, and thus the cars are uncertified, in violation of the Clean Air Act.”

Last week, FCA announced it would seek certification of its diesel-powered trucks and SUVs through a software fix. The Grand Cherokee and Ram 1500 models powered by the 3.0-liter diesel V-6 have been off the market since the automaker was first accused of employing potentially illegal devices. The fix would also be implemented on 2014-2016 model year vehicles to bring them into line with EPA and CARB regulations. The software fix would not hinder performance, FCA has stated.

The FCA situation is not identical to Volkswagen's crippling "Dieselgate" scandal. Where Volkswagen admitted to outright cheating the system with defeat devices, FCA and regulators seem to disagree over what the devices are capable of doing while remaining legal. FCA CEO Sergio Marchionne has denounced the accusations since January.

2014 Jeep Grand Cherokee EcoDiesel, New York City, Jan 2014

2014 Jeep Grand Cherokee EcoDiesel, New York City, Jan 2014

If a settlement isn't reached over the lawsuit, FCA could face penalties for violating air pollution laws totaling as much as $4.6 billion—$44,539 per affected vehicle.

In a statement, FCA said it is "disappointed that the DOJ-ENRD has chosen to file this lawsuit" and that it will "defend itself vigorously, particularly against any claims that the Company engaged in any deliberate scheme to install defeat devices to cheat U.S. emissions tests."

Through the complaint, FCA plans to continue its cooperation with the EPA and CARB to re-certify its diesel-powered trucks and SUVs.