Boeing engineers identified a fault with a pilot warning system on 737 Max aircraft in 2017, a year before the deadly Lion Air crash, the company said on Sunday, Agence France-Presse reported.

Boeing said that management was unaware of the issue until the October 2018 crash in Indonesia, which killed 189 people. The planes were not grounded worldwide until after another 737 Max, operated by Ethiopian Airlines, crashed in March, leaving a further 157 people dead.

According to Boeing, a supposedly standard piece of equipment that tells pilots about disagreements between angle-of-attack indicators - which measure the plane's angle vis-a-vis oncoming air to warn of impending stalls - did not activate unless an additional optional indicator was purchased by airlines.

That left airlines that did not buy the optional indicator, including both Lion Air and Ethiopian Airlines, without the safety feature.

It is not clear whether having the warning light would have prevented either the Lion Air or Ethiopian Airlines crashes, The Associated Press reported.

Faulty angle-of-attack, or AOA, indicator information may have played a role in both of the deadly crashes, causing the 737 Max anti-stall system to unnecessarily activate and push the nose down toward the ground even as pilots fought to maintain altitude.

"In 2017, within several months after beginning 737 Max deliveries, engineers at Boeing identified that the 737 Max display system software did not correctly meet the AOA Disagree alert requirements," the aircraft manufacturer said in a statement.

"The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature," it said. "Accordingly, the software activated the AOA Disa-gree alert only if an airline opted for the AOA indicator."

A Boeing review "determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation", concluding that "the existing functionality was acceptable until the alert and the indicator could be delinked in the next planned display system software update", the company said.

"Senior company leadership was not involved in the review and first became aware of this issue in the aftermath of the Lion Air accident."

Boeing is working to fix the software that pitched the planes' noses down based on faulty sensor reading, and to provide pilots with more information about the plane's automation. It hopes to win approval from the US Federal Aviation Administration and foreign regulators to get the Max flying again before summer is over. When it does, the company said, the sensor warning light will be standard, AP reported.

Nearly 400 Max jets were grounded at airlines worldwide after the Ethiopia crash, Xinhua News Agency reported.