Kia Motors Corp said on Friday it was recalling more than 507,000 vehicles in the United States because an electronic glitch may prevent airbags from deploying in a crash.
The recall follows an announcement in March by the National Highway Traffic Safety Administration (NHTSA) that it was investigating why some airbags had failed to deploy in Kia vehicles and its affiliate Hyundai after crashes in which four people were killed and another six were injured.
The two Korean automakers have now recalled nearly 1.1 million U.S. vehicles to address the issue.
NHTSA said in March that it was aware of six serious crashes in which airbags failed to deploy in frontal crashes, including four in 2011 model Hyundai Sonatas and two in 2012 and 2013 Kia Forte vehicles. The crash of the 2013 Forte occurred in Canada.
Kia's recall issued on Friday covers 2010-2013 Kia Fortes, 2011-2013 Kia Optimas and 2011-2012 Kia Optima Hybrid and Sedona vehicles.
The company said the airbag control unit may short circuit because they may be susceptible to electrical overstress, preventing the frontal air bags and seat belt pretensioners, which pull the driver and front seat passenger firmly back into their seats, from deploying.
The company said it does not yet have a fix, but is working with its supplier on the issue.
Kia spokesman James Bell said the company "is attempting to have a remedy by the scheduled owner notification date of July 27. If Kia does not have a remedy by that date or if any customer feels unsafe in his/her vehicle, we will provide a rental car until the repair has been completed."
Hyundai in February issued a recall for 154,000 U.S. Sonatas after non-deployment reports were linked to electrical overstress in the air bag control unit. In April, Hyundai recalled an additional 425,000 U.S. vehicles to address the same issue.
Hyundai said in March that it was aware of reports of two deaths in its vehicles, which occurred in head-on collisions at extremely high rates of speed.
NHTSA said the airbag control module under investigation was built by ZF Friedrichshafen AG, a German auto supplier.
ZF said on Friday that it has worked with Kia and "continues to cooperate and support NHTSA and its customers in the investigation."
The safety agency also said that electrical overstress appeared to be the root cause in a 2016 recall by Fiat Chrysler Automobiles NV of 1.4 million U.S. vehicles for airbag non-deployments in significant frontal crashes.
In March, NHTSA said it was investigating if other automakers used similar airbag control units and if they could pose a risk.
ZF said on Friday that each air bag control unit "is designed to a customer's particular vehicle and platform-specific specifications."