The Federal Aviation Administration said Wednesday that it had recently discovered a new problem with the 737 Max jet that Boeing must correct before the plane is returned to service.

In a flight simulator last week, FAA pilots tested erroneous activations of anti-stall software that pushes down the nose of the Max, two people with knowledge of the matter said. The software, known as MCAS, was involved in two crashes that killed 346 people.

In at least one instance, an FAA pilot was unable to quickly and easily follow Boeing's emergency procedures to regain control of the plane. The pilot