PSA Group, parent company to Peugeot and Citroen, is in the spotlight following an investigation into alleged cheating of emissions regulations.
According to French newspaper Le Monde, authorities found suspicious software had been allegedly used in almost 2 million vehicles sold by the car maker.
PSA has distanced itself from any use of fraudulent engine software. A spokesman directly denied the newspaper allegations, which sent the value of PSA stocks plummeting.
According to the report, PSA employed so-called “defeat devices” which restricted exhaust outputs of toxic nitrogen oxide (NOx) particles under regulatory test conditions. Outside of testing conditions, the group’s vehicles were said to exceed legal limits in real-world driving.
PSA became the fourth carmaker to be investigated by French prosecutors in February, following similar probes on Volkswagen, Renault and Fiat Chrysler.
At the time, a PSA engineering chief admitted emissions treatment in the group’s diesel engine was deliberately reduced at higher temperatures to improve fuel efficiency and emissions during open road driving, where NOx output is considered less critical than in built-up areas.
According to Le Monde, an internal PSA document obtained by investigators included discussion on the need to "make the 'defeat device' aspect less obvious and visible”.
However PSA insists there is nothing fraudulent or illegal about its engine calibrations. In fact, the car maker published results of real-world fuel figures earlier this year.
"PSA denies any fraud and firmly reaffirms the pertinence of its technology decisions," the company told newsagency Reuters on Friday.