Boeing has urged a number of airlines to replace a potentially faulty part on the wings of up to 133 of its 737 aircraft - months after the next generation versions of the planes were grounded after two fatal crashes.
The crisis-hit US company announced the action after an issue with "slat tracks" was disclosed by the Federal Aviation Administration (FAA).
Boeing said it had identified 21 planes most likely to have the parts in question.
It was advising airlines, which it did not identify, to also check an additional 112 planes.
The company said the slat tracks - found on the leading edge of an aircraft's wings - were manufactured by a third party and confirmed the tracks should be replaced before planes return to the air.
The FAA ruled the affected parts "may be susceptible to premature failure or cracks resulting from the improper manufacturing process".
It said that while a complete failure of a leading edge slat track would not result in the loss of an aircraft, a failed part could cause damage in flight.
Boeing said the same parts were also believed to have been used on 179 Boeing 737 MAX aircraft - the updated versions of the 737.
They were banned from flying by the world's aviation authorities after a crash in Ethiopia in March, which killed all 157 people on board.
That followed the deaths of 189 people on an Indonesian Lion Air flight when it crashed last October.
Both crashes involved the Boeing 737 MAX 8 aircraft and both have been blamed on problems with the flight-control software.
Air industry group IATA said last week it could be mid-August before fixes have been completed and the planes are judged airworthy again.
Boeing said it had seen no reported incidents involving the slat tracks and the remedial work was precautionary.
Kevin McAllister, president and chief executive of Boeing Commercial Airplanes, said: "We are committed to supporting our customers in every way possible as they identify and replace these potentially non-conforming tracks."