The National Highway Traffic Safety Administration is investigating complaints that the rear suspension frames on some Hyundai Sonata mid-size cars can rust and fail.
The probe affects about 393,000 cars from the 2006-through-2008 model years.
The NHTSA said that rust in the frame can cause control arm failures. A car's wheel hubs are attached to the control arms.
In documents on its website, the NHTSA said it had received six complaints of suspension failures. Three of the cases involved failures at highway speeds, but no crashes or injuries were reported.
In one case, the owner of a 2006 Sonata reported a loss of control in the rear wheels, which triggered the car's electronic stability control system.