The government auto safety agency said it is concerned that heat shields on certain Fords may rust and in time impede proper steering. That's based on five customer complaints and one car rollover injury suspected to have been caused by the problem.
The vehicles in question are 2004-2007 Crown Victoria, Grand Marquis and Marauder sedans.
The investigation does not constitute a recall, though it could result in one.
Ford said it would cooperate with the investigation and declined further comment.
General Motors has dominated the news on NHTSA investigations and recalls this year, but Ford has had its share of difficulties too.
The affected vehicles were model years 2008-2011.
The company also said 195,527 of its 2011 to 2013 Explorers had an "intermittent electrical connection" in their steering gears, which could result in a similar loss of power steering and risk of crash. In this case, however, drivers will be alerted by a chime and a message in the instrument cluster.
Earlier this month, Ford issued six recalls, including its Edge and Flex sport-utility vehicles, as well as some law enforcement vehicles -- for possible problems with power trains and other components.
Last summer, Ford was hit with a $17.35-million penalty by federal safety regulators for taking too long to recall nearly half a million Escape sport utility vehicles in 2012.