The Federal Aviation Administration said Monday that it had found numerous issues with production at Boeing and supplier Spirit AeroSystems.
Its audit found multiple instances where the companies failed to comply with manufacturing quality control requirements.
The FAA also said it found "non-compliance issues in Boeing's manufacturing process control, parts handling and storage, and product control."
The FAA has not detailed the corrective actions Boeing and Spirit must take, but sent a summary of its findings to the companies in its completed audit.
Spirit AeroSystems, which makes the fuselage for the MAX, said it is "in communication with Boeing and the FAA on appropriate corrective actions."
Boeing said that with the audit findings they now had a "clear picture of what needs to be done."
The FAA's audit was prompted by January's midair blowout on an Alaska Airlines 737 MAX 9.
Regulators later barred the planemaker from increasing production of the aircraft.
Last week, following a meeting with Boeing CEO Dave Calhoun, FAA Administrator Mike Whitaker said Boeing must develop a comprehensive plan to address "systemic quality-control issues" within 90 days.
Whitaker said the plan must incorporate results of the audit and findings from an expert review panel report released last week.
