The National Highway Traffic Safety Administration has launched an investigation of the 2013 Hyundai Sonata following two reports that the car's seatbelts failed to restrain passengers during collisions.
You might think that NHTSA would require more than two complaints before initiating a probe. Given the severity of this particular problem, however, NHTSA isn't waiting for more reports to filter in.
In each case, owners say that the front passenger-side seatbelt separated from the seat belt anchor. According to one report (sorry for the caps, that's the way NHTSA's website works):
"THE DRIVERS AND PASSENGERS SEAT BELT LOCKED AS INTENDED. THE PASSENGER SEATBELT FAILED AT THE POINT WHERE THE LAP RESTRAINT ATTACHES TO THE CAR RUNNING BOARD AND WILL NOT RE-ATTACH TO THE VEHICLE AFTER THE ACCIDENT."
In that incident, the passenger suffered an injury requiring medical attention.
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NHTSA's investigation is currently in the Preliminary Evaluation phase, during which NHTSA will work with Hyundai to identify the root cause of the problem. If there appears to be a persistent flaw in the Sonata, NHTSA may upgrade the probe to an Engineering Analysis, when it will try to recreate the problem in lab settings.
Depending on the results of those tests, NHTSA may request a full or partial recall of the Sonata. A full recall could affect as many as 313,000 vehicles in the U.S.