PSA Peugeot Citroen and the parent company of Mercedes-Benz are the latest car manufacturers to become embroiled in the emissions scandal.
Authorities raided the French car maker's offices on Thursday as part of its ongoing investigation into vehicle emissions, and on Friday, Daimler, which owns Mercedes-Benz said it was launching an internal investigation into the matter.
This week, Mitsubishi has seen its share price fall to a record low after it admitted falsifying the results of fuel economy tests on 600,000 of its vehicles. And it may find itself in further hot water after the company's president Tetsuro Aikawa said more vehicles may be involved.
Meanwhile, Volkswagen reached a settlement with US authorities over the emissions scandal. The US Department of Justice (DoJ) sued the German manufacturer earlier this year. VW was the first manufacturer found to have cheated emissions tests, using a so-called 'defeat device'. It has since recalled around 11 million cars worldwide. Volkswagen said it had agreed a "substantial compensation" package with the DoJ, which is expected to be revealed in June.
In a statement, PSA Peugeot Citroen said: "PSA Group confirms compliance of its vehicles in pollutant emissions in all countries where it operates.
"Confident in its technologies, PSA Group is fully cooperating with the authorities."
German manufacturer Daimler said it was launching its own investigation at the request of the DoJ.
Separately, the UK government's testing found "no evidence of car manufacturers, apart from the VW Group, fitting devices to defeat the approved emissions test programme", according to the Department for Transport.