Nissan Motor Co. President Hiroto Saikawa on Friday blamed a lack of communication between workers and managers as one of root causes for conducting improper checks of vehicles at the company's assembly plants using unqualified workers, in a practice that had been routine for more than 30 years.

Nissan said the first known occurrence of problematic inspections appears to date back to 1979 at its factory in Tochigi Prefecture. No problems concerning inspections were found at its Kyoto plant, it said.

Earlier in the day, the company submitted an investigation report to the transport ministry, which also cited a shortage of certified inspectors and poor awareness of compliance as reasons why unqualified workers had performed inspections of finished cars.