French carmaker Renault's efforts to clean up the emissions of some of its diesel vehicles after failing tests, in the wake of a pollution cheating scandal at Germany's Volkswagen, raise questions for the whole sector.

Q: Has Renault been cheating over its cars' emissions tests?

Both the company and French government deny this. Unlike VW which was forced to admit in September that it had fitted 11 million diesel engines worldwide with devices aimed at cheating emissions tests, Renault did not equip its cars with software capable of detecting test conditions in order to reduce the polluting emissions.

However some Renault diesel cars exceed the norms on polluting gas emissions such as nitrogen oxide, carbon dioxide (CO2) and nitrogen dioxide (NOx) when tested in real driving conditions, according to Ecology Minister Segolene Royal. Some foreign brands also failed the test, Royal said, but declined to name them. Renault says its cars have undergone European laboratory testing and conform with the norms.

Q: How is it possible to meet laboratory testing standards but to emit more pollution once on the road?

Emissions tests, which have the benefit of being the same for everyone, are carried out in conditions that do not fully reflect a car's daily usage. They maintain constant engine temperatures, only use gradual acceleration and are carried out without switching on energy-intensive electrical accessories.

The current NEDC (New European Driving Cycle) norm -- based on 1970s standards -- was criticised during the Volkswagen affair for being obsolete. New norms, called WLTP (Worldwide Harmonized Light Vehicles Test Procedure), which reflect much more closely normal daily driving conditions, are under discussion at the EU level and are expected to be adopted by 2017.

Q: What is Renault doing differently from its rival, France's number one carmaker PSA Peugeot Citroen, which seems to be in the clear?

Renault and Peugeot Citroen have gone down different technological roads to meet strict Euro 6 emissions norms, in force since 2014.

Renault currently uses a technology called a NOx absorber, or NOx trap, which captures nitrogen dioxide and then burns it.

The system is cheaper and simpler than a rival system called selective catalytic reduction (SCR), used by Peugeot and Citroen, but also less efficient in normal daily car use.

Renault, which already uses SCR for its trucks and vans, is to equip passenger cars with the technology over the medium term.

Q. What are the consequences for Renault, its shareholders and car owners?

The controversy could crimp the positive momentum the company had in 2015, when sales increased by 3.3 percent to over 2.8 vehicles. "We don't think that it will affect our image," said Thierry Koskas, Renault's Executive Vice President for Sales and Marketing, on Monday. But the company's shares have plummeted 15 percent in a week as investors remain jittery so long as the cost to Volkswagen of its major cheating of pollution controls remains unclear. For owners of Renault vehicles, the company has promised to unveil in the coming weeks a technical solution to bring actual emissions to permitted levels. It also disclosed 15,800 diesel versions of its Captur mini-SUV had already been recalled to correct a "calibration error" that led them to spew out more pollutants than allowed.

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