Government guidelines specify that workers who conduct these inspections must be “qualified” — though the authorities leave the details up to manufacturers. The car companies have the responsibility of deciding what training is required and of authorizing employees to work as qualified inspectors.

Nissan and Subaru stumbled on that step. They allowed some workers who had not completed training programs or who had not been formally authorized to be “final inspectors” to perform the checks.

The corner-cutting appears to have been long-established. Subaru said company rules designating who was allowed to inspect cars had been inconsistent with government guidelines for 30 years. Nissan says it is still investigating the scope and duration of its failure, but several news reports have suggested that it too may have been using unqualified inspectors for decades.

Independent specialists say it is unlikely that the inspection practices resulted in unsafe cars.

Automotive manufacturers and their suppliers perform multiple safety tests during development and production, and serious but hard-to-find faults — say, unstable chemicals inside an airbag inflater, which are believed to have caused the Takata hazard — are unlikely to be spotted by a limited, mostly external once-over.

Some commentators have suggested that part of the blame belongs with Japanese regulators, whose guidelines regarding inspectors appear simultaneously burdensome and vague. Cars made for export are not subject to the same rules, because governments outside Japan do not require final vehicle inspections to be carried out by workers with special training and qualifications.

“The checks are a formality, and it would not be an exaggeration to say they require no special know-how,” Hisao Inoue, a veteran Japanese automotive journalist, wrote on the website of the magazine Gendai Business after the initial admission by Nissan.

He said the inspection regulations should be reviewed, though he added that it was “absolutely wrong” for Nissan to have broken the rules.