Mazda Motor Corp, Suzuki Motor Corp and Yamaha Motor Co conducted improper fuel economy and emissions tests on their vehicles, Japan's transport ministry said on Thursday.
The report is the latest episode in a growing list of data falsifications in Japan that has tarnished the image of the country's manufacturing industry, known for high-quality, efficient production.
The findings were the results of internal investigations ordered at Japanese auto makers by the ministry after improper testing at Subaru Corp and Nissan Motor Co.
Representatives for Mazda and Suzuki confirmed they submitted reports regarding improper testing to the ministry but declined further comment. Yamaha Motor confirmed it carried out inappropriate testing.
"Regarding the emissions inspections... it is a fact that there were improper actions," a Yamaha spokesman said. "We sincerely apologise."
Mazda shares were down as much as 1.8 percent, their lowest in nearly four weeks, and Suzuki shares were down as much as 5.2 percent, its worst session since November 2016, versus a 0.5 percent fall in the benchmark Nikkei. Yamaha shares were down 4 percent.
In July this year, Nissan admitted it had improperly measured exhaust emissions and fuel economy for 19 vehicle models sold in Japan.
Kobe Steel, Mitsubishi Materials Corp and Toray Industries - all key suppliers of motor parts to global manufacturers - admitted to product data fabrication last year.
Many of the automakers, already hit by lackluster sales, have also been under pressure from US President Donald Trump's proposed tariffs on imported vehicles.