Shares of Mitsubishi Motors plummeted 15% today on news that the company manipulated test data for 620,000 minicars sold in Japan over the past three years. Mitsubishi is conducting an investigation to estimate the impact of the cheating on its business and to determine if overseas models are affected. The Japanese automaker indicated that the violation may result in it having to refund government tax rebates for the vehicles.

Discrepancies in mileage were discovered by Nissan, which sells rebadged versions of Mitsubishi models as the eK and DayZ for the domestic market. Nissan has suspended sales of the DayZ and DayZ Roox, pending further clarification from Mitsubishi.

The Japanese automaker is said to have overstated fuel efficiency ratings by 5 to 10 percent by altering tire and air resistance during testing. In addition, Mitsubishi separately admitted that is has used mileage testing methods since 2002 that are not compliant with Japanese standards.

Mitsubishi has been striving to restore confidence in the brand after being caught up in scandals involving flawed axles over a decade ago.