A series of simulator flights to test new software developed by Boeing revealed the error, according to one of the sources.
The latest versions of the popular Boeing jet were discontinued in March after two crashes – Lion Air Flight 610 and – Ethiopian Airlines Flight 302 – which killed 346 people.
While the crashes are still under investigation, preliminary reports showed that a new stabilization system pushed both aircraft into steep dive planes from which the pilots could not recover. The problem is known in aviation as the "runaway stabilizer trim".
Boeing announced that it was breaking the chain of events that led to both crashes by developing a software fix that would limit the effectiveness of this stabilization system.
In simulator tests, government pilots found that a microprocessor failure could push the nose of the aircraft towards the ground. It is not known if the microprocessor played a role in both crashes.
Testing the possible failure of the microprocessor in the simulators "made it difficult for the test pilots to recover within seconds," one of the sources said. "And if you can not recover in seconds, that's an unreasonable risk."
Boeing engineers are now trying to address the problem "The safety of our aircraft is a top priority at Boeing and we are working closely with the FAA to get the MAX back into service safely," a statement by Boeing said.
According to the sources, Boeing engineers are trying to determine if the microprocessor issue can be remedied by reprogramming software, or if it might be necessary to replace the physical microprocessors in each 737 Max aircraft.
An FAA spokesman would not confirm the specific problem. However, CNN said that "the FAA process is designed to detect and highlight potential risks, and the FAA has recently identified a potential risk that needs to be mitigated by Boeing."
Pilot training also updated
Boeing and the FAA are preparing additional training details for 737 MAX pilots, which may include additional simulator time, in preparation for the aircraft's recommissioning. the sources said.
Boeing and the FAA collaborate with European, Brazilian and Canadian civil aviation authorities.
The FAA is still actively considering whether more time-consuming and expensive simulator training will be required.
Gregory Martin, an FAA spokesman, said Wednesday the regulator "is following a thorough investigation and no deadline to get the Boeing 737 Max back in passenger service."
"The FAA will lift the ban on the plane if we believe it is safe," the spokesman said. "We continue to evaluate Boeing's software modification and are still developing the required training requirements."