U.S. regulators are reviewing some Hyundai and Kia recalls related to brake-safety units to determine if they should have been done sooner.

The National Highway Traffic Safety Administration’s Office of Defects Investigation is reviewing 16 Hyundai and Kia recalls conducted since 2016 that involve more than 6 million vehicles for potential antilock braking system module fires.

The recalls all involve the antilock braking system modules or hydraulic electronic control units made by the equipment supplier Mando.

The office said that the purpose of the review was “to evaluate the timeliness and scoping of Hyundai and Kia’s defect decision making and adherence with reporting requirements; and understand the varying defect descriptions and remedies between these recalls.”