Fiat Chrysler defeat devices allowed less pollution during tests, EPA says

— -- The U.S. government filed suit today against automaker Fiat Chrysler, alleging that it equipped more than 100,000 vehicles with so-called defeat devices to circumvent federal emission standards.

The software — installed on diesel-fueled Jeep Grand Cherokee and Ram 1500 in model years 2013 to 2016 — allegedly cause the vehicles' emission system to "perform differently and less effectively during certain normal driving conditions than on federal emissions tests," resulting in nitrogen oxide emissions above allowable levels during day-to-day driving, the Environmental Protection Agency said.

Fiat Chrysler maintains that its software was designed to detect not testing conditions specifically but temperature and other factors that could damage the engines if emission controls are activated.

The automaker "intends to 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," it said in a statement, adding that its officials have been working with the EPA to "clarify issues related to the company's emissions control technology."

The allegations against Fiat Chrysler come on the heels of a huge settlement with Volkswagen, which in March pleaded guilty to intentionally thwarting EPA standards with different defeat devices installed in more than half a million cars in the United States. Volkswagen agreed to pay $4.3 billion in criminal and civil penalties.

ABC News' Daniel Steinberger contributed to this report.