In some cases, drivers reported that a warning message appeared on the instrument panel before the power steering quit. At times, restarting the SUVs corrected the problem, several drivers reported.
An investigation could lead to a recall. NHTSA said it is looking into how often the problem happens and what's causing it.
Ford is cooperating with NHTSA in the probe. Anyone who has a concern about their vehicle should contact their dealer, company spokeswoman Marcey Zwiebel said.
The Explorer, once a hulking family-hauler that helped launch the truck-based SUV boom in the early 1990s, was redesigned for the 2011 model year. The new version is based on car rather than truck underpinnings and is more efficient and maneuverable than the old one.
The new vehicle started arriving at dealers late in 2010. The company sold more than 135,000 Explorers in 2011.
Ford shares edged up 3 cents to $10.30 in morning trading Friday. They are down 20 percent from $12.90 in mid-March.