The probe centers on "failure of the front lighting control module resulting in loss of headlights while driving," NHTSA said in the problem description posted on its Web site on Friday.
NHTSA said there are 3,609 complaints regarding loss of headlights in these vehicles. Some of the complaints were reported by consumers to NHTSA, while Ford provided data indicating it has received 3,092 complaints of failed headlights.
NHTSA said it has identified 15 crash allegations, one injury and no fatalities.
One driver was injured when the car struck an unseen object in the road, "jerking the steering wheel from their hands and injuring their shoulder," NHTSA said.
Other drivers reported incidents of running off the road due to a lack of headlights; one driver reportedly struck a deer.
Ford spokeswoman Kelli Felker told Edmunds the automaker will cooperate with NHTSA on the investigation.
NHTSA has opened a preliminary evaluation into the matter "to further evaluate the scope, frequency and consequence of the alleged defect."
Such an action sometimes precedes a vehicle recall, but the vehicles have not been recalled at this point.
Edmunds says: If you own one of these cars and are experiencing a problem, it's best to contact your Ford dealer now.