Boeing has said "a significant number" of 737 MAX range will be produced more slowly than planned, meaning "lower near-term deliveries" to airlines.
The Seattle-based manufacturer said it was told by a supplier, which it did not name, that the installation of rear fittings was not to standard
"This is not an immediate safety of flight issue and the in-service fleet can continue operating safely," Boeing said.
Spirit Aerosystems, the supplier, later said it is "working to develop an inspection and repair for the affected fuselages".
Boeing shares fell 5% upon the announcement of the latest difficulty it has faced with the MAX, which was grounded worldwide after fatal crashes in Indonesia and Ethiopia in 2018 and 2019.
Boeing had to pause deliveries of its 787 Dreamliner in early 2023, before US regulators in March cleared it to resume production.
Boeing reported in mid-April that it delivered 130 airplanes in the first three months of 2023.