Grounded Southwest Airlines Boeing 737 Max 8 aircraft are shown parked in Victorville, Calif., on March 26. (Mike Blake/Reuters)

Boeing’s 737 Max fleet was supposed to have a standard cockpit alert that would warn pilots when sensors outside the plane were feeding in incongruous data, a problem that contributed to crashes in Indonesia and Ethiopia that killed 346 passengers and crew members.

But in fact, that safety alert only worked in aircraft with an optional feature, the manufacturer said on Monday.

“Boeing did not intentionally or otherwise deactivate the disagree alert on its MAX airplanes,” the company said in a statement. “The disagree alert was intended to be a standard, stand-alone feature on MAX airplanes. However, the disagree alert was not operable on all airplanes because the feature was not activated as intended.”

The revelation is likely to add more regulatory scrutiny to Boeing as it tries to fix the software implicated in the crashes and gain the approval it needs from the Federal Aviation Administration to end a global grounding of its signature passenger aircraft.

The alert in question is meant to warn pilots if the plane is being fed faulty data from angle-of-attack sensors, which measure the pitch of the plane’s nose. In both the Lion Air crash in Indonesia on Oct. 29, 2018, and the Ethiopian Air crash on March 10, Boeing has acknowledged that malfunctioning sensors led anti-stalling software called the Maneuvering Characteristics Augmentation System (MCAS) to override pilots.

On Sunday, the Wall Street Journal and others reported that neither the FAA nor Southwest Airlines, Boeing’s largest 737 Max customer, knew that the safety alert was deactivated.

Boeing’s statement comes in response to those reports. The manufacturer said the safety warning system had unintentionally been tied into an optional feature and wouldn’t work without it.

“The disagree alert was tied or linked into the angle of attack indicator, which is an optional feature on the MAX. Unless an airline opted for the angle of attack indicator, the disagree alert was not operable,” the company said.

The revelation came hours after Boeing’s chief executive faced pointed questions from investors and reporters in Chicago on Monday. CEO Dennis Muilenburg said the company “owns” responsibility to up safety on the 737 Max but declined to say that it had released the plane with design flaws, reported The Washington Post’s Douglas MacMillan. Instead, he declared the recent crashes the result of a “chain of events,” with Boeing’s malfunctioning sensors and software just one piece of the problem.

“When it comes to safety, there are no competing priorities,” Muilenburg said at the meeting.

That explanation didn’t hold water for protesters outside Muilenburg’s speech, including relatives of those who died in the crashes.

“Their response has been a farce,” Tarek Milleron, whose 24-year-old American niece died in the Ethiopian Airlines crash, told The Post. “It’s a hollow denial."

Earlier on Monday, Boeing secured a new $1.5 billion line of credit, The Post reported, a sign of the firm’s tentative financial outlook with all its 737 Max sales on pause.

The company said earlier this month that it’s testing a fix to the MCAS software in the hopes of getting FAA permission to resume 737 Max commercial flights, which were grounded around the world in the weeks after the crash outside Addis Ababa.