Boeing Co. has identified a new software flaw in the grounded 737 Max which will require additional work before the plane can be returned to service.

The company (BA) has notified FAA and is notifying customers and its suppliers about the issue, it said in an emailed statement. Boeing’s best-selling jet was grounded on March 13 after two fatal crashes involving a flight-control system.

The issue involves how a software system on the plane checks to ensure that data it is receiving from other monitors are functioning properly, said a person familiar with the issue who wasn’t authorized to speak about it.

“We are making necessary updates and working with the FAA on submission of this change, and keeping our customers and suppliers informed,” Boeing said in its statement. “Our highest priority is ensuring the 737 MAX is safe and meets all regulatory requirements before it returns to service.”