US regulators said Boeing must tackle a new "potential risk" for the Boeing 737 MAX on Wednesday and continue to tarnish the resumption of service after two deadly crashes.
The problem encountered in testing the FAA simulator involved the ability of pilots to quickly restore control of the aircraft when an automated air handling system pushes the aircraft down, a person familiar with the matter said.
The Federal Aviation Administration "will abolish the prohibition regulation of the aircraft, if we consider it safe," the agency said in an e-mail.
"The FAA has recently identified a potential risk to Boeing."
According to Boeing, the software update for the 737 MAX, which was developed in the last eight months, is currently not available.
"The Boeing Company agrees with the FAA's decision and solicitation and is working on the software required to answer the FAA's request," Boeing said in a letter of filing.
"Boeing will not offer the 737 MAX for certification by the FAA until we have met all of the MAX certification requirements and safe recommissioning."
In a Wednesday media release, the company said the resolution of the "condition" would reduce the workload of pilots by "taking into account a potential source of unauthorized stabilizer movement".
Boeing's shares fell 0.7% to $ 372.50 after the close.
The Boeing global fleet of 737 MAX aircraft has been down since mid-March after two accidents involving 346 people.
An important step in the certification process is an FAA test flight scheduled for at least the week of July 8, a person familiar with the matter said.
Even before this latest issue surfaced, the prospects of getting the planes blown up were uncertain, partly because the FAA wanted other regulators to approve the plane's re-entry shortly after the US agency.
Some regulators have advocated calling for simulator training for pilots on the 737 MAX, an idea that was advocated last week by retired pilot Chesley "Sully" Sullenberger at a congressional hearing.
Pilot simulator training would be costly and time consuming to retrieve, in part because only four 737 MAX simulators are currently on the market. However, some panelists at the hearing mentioned that simulators might be used on previous Boeing 737 models.
US airlines such as American Airlines and Southwest Airlines have recently postponed their timeframe for the re-flight of aircraft until the end of the summer.
Much of the attention since the crash of Lion Air and Ethiopian Airlines has focused on the Maneuverability Augmentation System (MCAS).
In both MAX crashes, the MCAS sharply pointed down the aircraft due to a faulty sensor gauge, which made it difficult for pilots to control their aircraft after takeoff, according to preliminary crash reports.