Aluminum and copper products used in millions of Toyota and Honda vehicles may have been shipped without meeting grade specifications.

Honda and Toyota are facing a potentially significant headache over a scandal involving metals supplied by Kobe Steel.

The Japanese metal supplier recently admitted to selling products with falsified inspection certificates. Some metals were consequently shipped without meeting specifications required by customers.

"The Company has been carrying out technical verifications with its customers on the impact of the nonconforming products on quality (including safety) of the end products," Kobe Steel said in a statement.

The initial disclosure suggested the scandal was limited to products sold starting in September 2016, however follow-up statements suggest the misconduct could trace back an entire decade.

Automakers are presumably launching their own investigations to determine if components built from Kobe-sourced metals perform to their original expected specifications. Suspect parts may have been used in millions of vehicles sold by Honda, Toyota and other automakers in Japan and beyond.

Honda's internal investigation has already uncovered one potentially affected aluminum product used to build vehicle doors and hoods, though the company has not yet determined if the metals fail to meet the original specification or impact safety.

The scandal represents yet another blemish for once-lauded Japanese manufacturing, emerging before the dust has settled from the Takata defect crisis.