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. It includes gas-electric hybrid versions of the cars.
According to a class-action lawsuit filed in June about the matter, the problem could affect more Ford models, including the compact Focus.
The National Highway Traffic Safety Administration says it has received 508 complaints alleging that the midsize cars lost power-assisted steering, causing increased steering effort. Four complaints say the problem caused crashes, but no injuries were reported.
Ford says it's co-operating with the investigation and that anyone experiencing power steering problems with their vehicles should contact their dealer.
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.
NHTSA says it will check the scope and frequency of a problem. It could seek a recall.
In one of the complaints filed with NHTSA, from August of 2013, a woman said she was driving her 2011 Fusion to work when she tried to turn right into a driveway and the power steering failed. The woman said she nearly hit another vehicle that was leaving the business. "Brakes were applied sharply and every bit of the 120-pound female driver's strength was needed to manually steer the vehicle into the parking lot," the complaint stated.
Another complainant wrote that in July of 2013, the power steering of his 2012 Fusion failed as his wife was getting ready to leave the house to take their children to day care. "The scariest part is what could have happened if my wife had been driving with the kids and trying to make a turn?" the man asked.
Some drivers complained that dealers couldn't reproduce the problem and they were left to drive a car that hasn't been repaired. Others complained that their cars were out of warranty and the repair cost $1,400 to $1,900.
People who file complaints are not identified in the NHTSA database.
The investigation follows a class-action lawsuit filed in San Jose, California, in June alleging that electronic power steering systems on the 2010 through 2014 Fusion and 2012 through 2014 Focus compact cars are defective. The lawsuit alleges that Ford has received hundreds of complaints about the systems, yet it hasn't disclosed to owners that the systems can fail.
"As a result, drivers of the defective vehicles experience markedly increased steering effort, leaving them unable to control their vehicles," the lawsuit states.
Ford spokeswoman Kelli Felker said the company does not comment on pending litigation.
Shares of Ford Motor Co. slipped 3 cents to $14.56 in morning trading Monday. Its shares are down 14 per cent over the past year.