The National Highway Traffic Safety Administration is opening an investigation into the 2010-2012 Ford Fusion, Fusion Hybrid and Lincoln MKZ, the 2012 MKZ Hybrid and 2011 Mercury Milan because the agency has hundreds of complaints of electric power steering failure in these models.

According to NHTSA’s data, it has received 508 complaints of the power steering allegedly suddenly malfunctioning and resulting in increased effort to turn the wheel. In four incidents, these failures resulted in loss of control and crashes. According to the reports, in some cases a power steering warning message came on as the fault occurred, and other times the system restored itself by turning off and then restarting the vehicle. NHTSA said it has also received further field report data from Ford, but that information hasn’t been publicly released yet.

For the moment, there isn’t a recall for this failure on these models. NHTSA is just investigating them to “assess the scope, frequency and safety consequences of the alleged defect,” and the agency says an estimated 938,000 vehicles could potentially be affected.