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.
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.
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.