PARIS -- French prosecutors have opened a formal investigation into suspected emissions test-cheating by PSA Group.
The Paris prosecutor's office opened the probe on April 7 after receiving a report from the French Economic Ministry's fraud division, a spokeswoman for the prosecutor said Monday.
The fraud would have made the cars dangerous to human health, she said. Three judges are assigned to lead the investigation.
PSA said on Monday it had "taken note" of the prosecutors' decision to open a full investigation, reiterating that its vehicles comply with all regulations in countries where they are sold.
PSA's "vehicles have never been equipped with software or any device that would have allowed them to detect" testing, a spokesman for the carmaker said.
In February, PSA became the fourth carmaker to be referred to prosecutors by France's DGCCRF consumer fraud watchdog over suspected test manipulation in the wake of the Volkswagen Group diesel emissions-rigging scandal.
Following VW's admission in 2015 of cheating on U.S. diesel-emission tests, France began randomly testing vehicles to check differences between lab results and real-world emissions.
The tests showed on-road NOx and CO2 emissions above regulatory limits and widespread use of devices that reduce exhaust treatment in some conditions. Automakers have broadly invoked a European legal loophole allowing such software for safety purposes or engine protection.
So far VW, Renault, FCA and PSA have been referred for possible prosecution by the DGCCRF.
Under French law, carmakers found to have cheated on the vehicle-certification process can be fined as much as 10 percent of their average revenue for the three years prior to the incident. Executives risk up to two years in jail and 300,000 euros ($327,000) in fines.
PSA diesels under the current Euro 6 standard have cleaner emissions than many mass-market rivals, thanks to their standard deployment of costly selective catalytic reduction (SCR).
But in the testing program last year, five PSA vehicles of the last Euro 5 diesel generation emitted significantly higher NOx in motorway driving conditions when engine temperatures were increased.
By design, the cars' so-called EGR emissions treatment is deliberately reduced at higher temperatures to improve fuel efficiency and CO2 emissions in out-of-town driving, where NOx and particle output is less critical, PSA engineering chief Gilles Le Borgne said in February.
The company last year began an independently certified real-world CO2 emissions-testing program and began publishing results measured for all its vehicles on the road.
Bloomberg contributed to this report