Japanese car maker Suzuki has apologised for improper road tests on some of its models but denies it illegally falsified mileage numbers.
Suzuki's shares fell in Tokyo trading on Wednesday amid Japanese media reports the company might have cheated on its mileage data.
Earlier this month, Mitsubishi said it had systematically falsified mileage on several models to inflate them.
Nissan then bought a 34% stake in Mitsubishi for 237bn yen.
Nissan chief executive Carlos Ghosn said this was a good deal because Mitsubishi's stock had plunged more than 40% after the scandal broke.
Suzuki, whose line-up focuses on mini-cars, or models with small engines that make them eligible for tax breaks in Japan, said the dubious tests did not affect models sold abroad.