Government sources on Wednesday said the Nissan Qashqai SUV is the worst offender, prompting authorities here to consider a ban, a recall or a hefty fine for the Japanese automaker. The government is also considering a recall for the other 13.

Nitrogen oxide gases are formed in all combustion engines and are a major source of air pollution, but they are especially bad in cars that run on cheap and dirty diesel.

Fourteen out of 16 diesel cars sold in Korea emit three to 10 times more nitrogen oxides than permitted, an investigation reveals.

According to the Environment Ministry, the National Institute of Environmental Research in January tested the emissions of 16 diesel cars made by five domestic automakers and 11 foreign carmakers. All supposedly meet the Euro 6 emission standards.

But the test, which involved driving them for 3,000 km and inspecting their emissions in various conditions like city or highway traffic and in different temperatures, told another story.

Only the BMW and Jaguar Land Rover diesel cars performed well by emitting less than 80 mg of nitrogen oxide for every 1 km. The other 14 models emitted anywhere between three to 10 times more nitrogen oxide.

Hyundai's diesel cars emitted five to six times more nitrogen oxide than permissible, about average among the cars tested.

Investigators found that the exhaust gas recirculation devices on the 14 cars were programmed to stop operating when external temperatures reached above 30 or below 20 degrees Celsius, causing excessive emissions of nitrogen oxides.

One source said, "EGR stopped functioning when the air conditioner was turned on or the cars accelerated or stopped suddenly. This is prohibited by law, but investigators believe the cars were deliberately programmed that way."

In November last year, Volkswagen was ordered to recall the popular Tiguan SUV because its EGR was programmed to stop working under high or low temperatures, and the German automaker was banned from selling the SUV in Korea and slapped with fines.

But the Nissan Qashqai is apparently worse. It is popular in Europe, and around 2,000 have been sold in Korea since 2014.