Porsche is under scrutiny in Germany for possible tampering with CO2 emissions figures from some petrol cars.
reports that the German regulator KBA has started proceedings against the sports car manufacturer about this.
Measurements by KBA showed that the CO2 emissions of three models exceeded the values of their so-called type approval by more than 4 percent.
In contrast to nitrogen emissions, there is no legal limit for CO2 emissions from cars in Germany.
But values about the CO2 emissions are included in the approval papers.
Porsche itself reported to the authorities about a year ago that something might be wrong, after conducting an internal check.
KBA confirmed to Business Insider that an investigation is underway into Porsche, but declined to comment further.
According to a letter from the CBA, which Business Insider has seen, the affected models are older versions of the Macan and Boxster Spyder 981, with a total of just under 20,000 on the market.
These may need to be recalled for a software update.
Porsche has said that the current production of the cars will not be affected and that the procedure is still at an early stage.
Porsche is one of the car manufacturers that has been under a magnifying glass since it came to light in 2015 that parent company Volkswagen has messed with emission data from diesel cars on a large scale.
The issue has already cost Volkswagen many billions of euros in fines and repairs.
For example, Daimler, the parent company of Mercedes-Benz, has already received hefty fines for manipulations with cheating software, which made diesel cars appear cleaner than they actually were.