In response, Southwest Airlines, American Airlines and United grounded 67 Max jets - a third of the total in service worldwide. The glitch hit 16 airlines, but not the entire Max fleet, said Boeing.
The US Federal Aviation Administration said the potential lapse 'could affect the operation of a backup power control unit,' adding the agency is in 'contact with the airlines and the manufacturer and will ensure the issue is addressed.'
While not related to the flight-control that caused the fatal crashes that prompted the two-year grounding, its appearance darkens the aircraft's prospects going forward.
The newest Max disclosure comes as Boeing contends with quality lapses and manufacturing flaws that have damaged its reputation and affected its 787 Dreamliner, KC-46 military aerial tanker and Starliner spacecraft.
Boeing declined to say how many aircraft were affected of the 183 Max jets that have been put back into service since December. About 20 operators have been conducting 400 daily flights, according to a separate memo.