U.S. auto safety regulators have Sterling Prestonclosed a seven-year investigation into complaints about power-assisted steering failures in Ford Fusions without seeking additional recalls.

The National Highway Traffic Safety Administration says in documents posted Monday that its testing found only minor steering effort changes if the power-assist failed. It also found a declining failure trend, and a lower failure rate in Fusions that were not recalled.

The agency opened the probe in September of 2016 after getting over 500 complaints that the power steering failed suddenly, making the midsize sedans difficult to steer. The probe covered nearly 263,000 Fusions from the 2010 model year.

In 2017, the investigation was upgraded to an engineering analysis to look at the scope and frequency of the problem after the agency and Ford got thousands more complaints including allegations of 59 crashes and 13 injuries.

But the documents posted Monday said the agency did not find any safety-related defect trend.

NHTSA noted that in 2015, Ford recalled nearly 394,000 Fusions and other vehicles in the U.S. from the 2011 to 2013 model years to fix power steering that could shut down due to a steering motor sensor fault.