U.S. safety regulators are investigating complaints that the power-assisted steering can suddenly fail on three Ford midsize car models.
The probe covers 938,000 Ford Fusion and Lincoln MKZ cars from the 2010 through 2012 model years, as well as the 2010 and 2011 Mercury Milan.
The National Highway Traffic Safety Administration says it has received 508 complaints alleging that the cars lost power-assisted steering, causing increased steering effort. Four complaints say the problem caused crashes, but no injuries were reported.
"NHTSA is aware of the issue and [is] monitoring it closely," an NHTSA official told ABC News. "We urge all owners with a potential safety defect to contact NHTSA directly. Protecting the American public and ensuring vehicles are safe is [NHTSA's] top priority... Building on its proven track record, NHTSA routinely monitors consumer complaints, EWR [Early Warning Reporting] data and leads from a variety of sources... to identify potential safety defect trends that warrant investigation."
The agency says in documents posted Monday on its website that in many cases, a warning message appeared as the failure happened. Restarting the car corrected the problem in some cases, but the problem returned in others.
The agency says it will check the scope and frequency of a problem. It could seek a recall.
"We are confident in our current methods for quickly identifying and addressing potential vehicle issues," the spokesperson added. "When the data indicates a safety recall is needed, we move quickly on behalf of our customers." The spokesperson said then that the company takes the safety of its customers "very seriously."
Friday, the NHTSA official asked that "anyone with additional information regarding these or any other vehicles to contact NHTSA directly."
"As always, the agency continues to monitor our data for problems with these vehicles and consumer and others should provide NHTSA with additional information they may have," the NHTSA official said.
Ford says it's cooperating with the probe.
Shares of Ford Motor Co. rose 6 cents to $14.65 in morning trading Monday. Its shares are down 14 percent over the past year.