NHTSA looked into 163 complaints about the failure of mats in the front passenger seats that detect whether a child is there but decided not to seek a recall of the vehicles.
Children and small-statured adults could be harmed by the inadvertent deployment of the passenger frontal airbag.
Federal safety regulators said they would not seek a recall because the vehicles "experience a comparatively low complaint rate for this issue given the number of years the vehicles have been in service."
In addition, Kia extended the mat warranty program, providing free replacement for 15 years regardless of vehicle mileage.
Kia will send owners a letter notifying them of the potential hazards posed by failure of the occupant-classification system, along with the terms of the extended warranty.
"Consumers whose airbag warning light is illuminated will have the opportunity to have their vehicle diagnosed and, if a fault is indicated, have the OCS sensor mat repaired at no charge," NHTSA said.
NHTSA said the closing of the investigation does not constitute a finding that a safety-related defect does not exist, and the agency reserves the right to take additional action if warranted by new circumstances.
Edmunds says: Owners of these vehicles do not need to take any action at this point, unless they have a concern about the occupant-classification system. Kia dealers can help those with immediate concerns.