Nissan Motor has become the latest Japanese automaker to admit to falsifying product-quality data, dealing a further blow to Japan Inc.’s reputation for dependable quality.

An internal review of emissions and fuel economy tests at Nissan’s production plants in Japan showed that company inspectors used “altered measurement values” on emissions inspection reports, the company said in a statement on Monday. The tests also “deviated from the prescribed testing environment,” it said.

The review found that all models complied with Japanese safety and emissions standards, it said. The exception was the Nissan GT-R, a two-door sports car, which the company produces too few of to comprehensively review its record, said Nick Maxfield, a Nissan spokesman, in an email. The company said the falsification problems ultimately did not affect fuel-economy findings.

Nissan said that it had already started investigating the falsifications and that it had retained a Japanese law firm, Nishimura & Asahi, to lead the effort. The investigation is likely to take one month, Mr. Maxfield said.