The probe announced Friday covers about 394,000 midsize cars from the 2006 through 2008 model years.
The National Highway Traffic Safety Administration says it has received 83 complaints about the problem. The agency says a sensor inside the seat belt buckle might fail. This can cause the air bags to malfunction or not inflate if there's a crash.
The problem also can affect the mechanism that tightens the seat belts before a crash. The problem can happen in either the driver or passenger buckles. In most cases the air bag warning light came on.
Investigations can lead to recalls but none has been issued so far in this case.
Hyundai will co-operate with the NHTSA in its investigation, company spokesman Jim Trainor said in an email.