U.S. safety regulators have opened a preliminary investigation into claims that Hyundai Motor Co.’s best-selling car in America, the Sonata sedan, may have steering problems.

National Highway Traffic Safety Administration officials opened the preliminary investigation into the 2011 Hyundai Sonata Friday.
There have been no injuries or accidents reported to NHTSA or Hyundai regarding the steering issue, government and company officials said.

NHTSA said it g issue could be present on 16,300 Sonata sedans built in one  month at Hyundai’s plant in Alabama. NHTSA said Sonatas cited in its reports each had fewer than 600 miles driven on them.

NHTSA will investigate consumer complaints that “the steering shaft allegedly decoupled from the hand wheel resulting in a complete loss of steering capability,” according to NHTSA.

Hyundai spokesman Jim Trainor said two Sonata customers brought their sedans to Hyundai dealerships complaining of steering problems. Those cars were fixed and returned to the owners, he said.

Sonata is a key to Hyundai’s success in the U.S. market. Through July, Sonata sedans accounted for 46 percent of Hyundai’s car sales this year.

The Sonatas for the U.S. and Canadian markets are built in Alabama. Sonatas sold outside those markets are built in South Korea, where Hyundai and its sister company Kia are headquartered.

Hyundai’s U.S. sales are up 23 percent from last year through July, and U.S. sales for the Sonata were nearly 18,000, according to Autodata. In 2009, Hyundai was the only major automaker to increase sales and market share.

Kia’s share of the U.S. market so far this year is 3.1 percent.

In 2008, Hyundai sales accounted for 3 percent of the U.S. market. Through July this year, its market share is 4.6 percent.

NHTSA officials had no comment beyond papers filed on its site.

Last month, the agency opened a similar preliminary investigation  for the Kia Soul after a driver complained of a total loss of steering and limited braking in a car that was two months old and had about 4,300 miles  on it.

The Soul is the rival to the Scion xB from Toyota’s brand targeted at younger drivers.

Kia sales through July were up 16 percent, outperforming the U.S.  market’s 14-percent gain.