After 161 complaints of engine fires, the National Highway Traffic Safety Administration (NHTSA) is intensifying its probe into Hyundai and Kia vehicles, according to a report Monday from the Associated Press.

Both automakers are based in South Korea and Hyundai Motor Co. owns a share of Kia Corp.

The engine fires, which were first reported six years ago, affect more than 3 million vehicles. They involve the model years 2011 to 2016. Some of the Hyundai and Kia vehicles were recalled.

As many as eight recalls have been issued for a series of engine problems that date back to September 2015 by Hyundai and Kia.

NHTSA has yet to respond to a request for comment from International Business Times.

NHTSA is reportedly looking to evaluate whether the recalls covered enough vehicles and is monitoring the effectiveness of the recalls.

The engine fires were linked to the automakers’ Theta II GDI, Theta II MPI, Theta II MPI hybrid, Nu GDI and Gamma GDI engines, which affected the 2011 to 2016 Hyundai Sonata, Santa Fe and Elantra and 2011 to 2016 Kia Sorento, Rio, Optima and Soul models.

The consumer complaints to NHTSA indicated that three people suffered from eye and burn injuries as a result of engine fires but did not require medical treatment, according to the AP.

In November 2020, NHTSA fined Hyundai and Kia a total of $137 million for failure to recall more than 1 million vehicles promptly that had engines that could fail, citing a previous probe involving recalls of models that dated back to 2011.

Kia paid $27 million in addition to $16 million for safety performance measures, with $27 million deferred if it met certain safety conditions set forth by NHTSA. Kia’s consent order is for two years, with NHTSA having the option to extend it another year if warranted.

Hyundai paid $54 million, with $40 million for safety performance measures and $46 million deferred if specific safety conditions are met, the agency said. Hyundai’s consent order is for three years, with the option to extend if NHTSA sees fit.

Since that time, the automakers underwent product enhancements that impacted 3.7 million vehicles with a software install that would alert drivers if the engine had a failure, the AP noted.

It is unclear if the engineering analysis from NHTSA has the potential to prompt more recalls.

In 2014, the U.S. government fined Hyundai and Kia a combined $300 million for overstating vehicle fuel economy standards on 1.2 million cars.

Hyundai’s advertising arm will showcase smart driving sunglasses at CES next week. Reuters/Stephen Lam