As The Los Angeles Times reports, the FAA is worried about potential problems to the aircraft's horizontal flight-control surfaces, which could result in a "possible loss of control of the airplane."
The paper adds:
"The agency said faulty parts could result "in a significant pitch upset" and cause pilots to lose control of the aircraft.
"The parts in question have not been linked to any accidents involving Boeing's popular jet, but nonetheless, regulators have ordered inspections of the elevator systems over the years.
"At least one airline, United Airlines, objected to the FAA action, arguing that service bulletins issued by Boeing in 2007 and 2008 have addressed many of those problems. The agency, however, rejected those arguments."