Shares in Mitsubishi Motors Corp. 7211, fell 15% on Wednesday in Tokyo after it emerged that the Japanese automaker's cars didn't pass a fuel economy test and misconduct was a possibility. Later Wednesday in Tokyo, the company admitted to cheating on fuel economy tests for more than 600,000 vehicles, according to media reports. Mitsubishi said its employees faked test data tied to 157,000 of its own cars and 468,000 vehicles made for rival Nissan Motor Co. 7201, +1.31% , the reports said. The misconduct reportedly came to light after Nissan found problems in the testing data. Investors are sensitive to news about testing snafus following Volkswagen AG's VOW3, -3.41% emissions scandal that erupted last year.