Inspectors were instructed to fake data if real measurements did not meet certain quality control standards.
Subaru has confirmed that its inspectors altered fuel efficiency and emissions data for hundreds of vehicles or more dating back to 2012.
After initially uncovering evidence of data manipulation, the company performed an internal audit with a sample group of nearly 7,000 vehicles that could be measured again. Data were then compared against figures stored in measurement equipment.
The inquiry confirmed "inappropriately altered" data for 903 vehicles, representing more than one in eight vehicles in the sample group.
"Inspectors ... were instructed by their seniors that, if results for each vehicle did not meet such standards, measurement values should be altered to those that meet such standards, and, according to such instructions, the inspectors altered measurement values," Subaru said in a statement.
Confusingly, despite confirming data alteration, the Japanese automaker claims all 903 vehicles met the company's internal quality control standards.
"It should be noted that alterations were made not only to make results better, but also to make them worse," the company added.
The audit was only able to go back to 2012, the latest year with reference values stored in measurement equipment. The falsifications are believed to date back to 2002, however.