The transport ministry plans to start road trials for diesel vehicles after the amounts of nitrogen oxide emitted from four of six models exceeded the maximum-allowable level in a recent test.
Transport minister Keiichi Ishii told a news conference on Friday that no software that manipulates emission levels was found in a recent evaluation of the six models that was conducted following the admission by German automaker Volkswagen AG that it cheated on emissions tests.
But the four models from Toyota Motor Corp., Nissan Motor Co. and Mitsubishi Motors Corp. emitted NOx two to 10 times the maximum level allowed in bench tests, according to the ministry.
That was because the system that reduces exhaust emissions stopped functioning during low winter temperatures outside, the ministry said.
The NOx levels were mostly within the limit for the remaining two models, both from Mazda Motor Corp.
At present, vehicles are given the green light unless their NOx emissions exceed the limit in bench tests.
The results prompted the ministry to introduce the road tests for diesel vehicles before their release.