According to Bloomberg, the lawsuit alleges that Chrysler, together with its diesel engine partner Cummins, has concealed the nitrogen oxide output of certain Ram vehicles produced between 2007 and 2012. The NOx pollutants were meant to be broken down in a process called regeneration in the truck's NAC system, or NOx Absorption Catalyst, which predated the 2013-introduced SCR, or Selective Catalytic Reduction system. By design, the NAC captures and stores NOx emissions, converting them to nitrogen and oxygen through a catalytic process. The lawsuit claims the Cummins engine's system has a limited capacity to store the emissions, and as a result the pollutants escape, increasing emissions, worsening fuel consumption and wearing down the catalytic converter.
The later, cleaner SCR system uses a urea-water injection, and it gradually replaced the NAC on Cummins 6.7-liter engines, as it was first implemented in 2011 and made standard in 2013. As Bloomberg notes, the model years of Ram trucks involved in the lawsuit predate the earliest Volkswagen "Dieselgate" models by two years. The lawsuit, filed on behalf of 500,000 truck owners, accuses Chrysler and Cummins of fraud, false advertising and racketeering.
As an underlying motive, the filing mentions a 2001 change in EPA emissions standards. Announced to become effective in 2010, the EPA requirements drove Chrysler and Cummins to try and reach those already by 2007. However, the NAC system is said to have fallen short of these goals, and the filing claims that Chrysler and Cummins chose to "rig" the engines instead.
The affected vehicles predate the 2014 merger of Chrysler and Fiat. FCA US has released a statement regarding the lawsuit, saying it will contest the lawsuit "vigorously".