The National Highway Traffic Safety Administration also said that it was satisfied with a plan by General Motors (GM.N) to address problems that triggered fires in Volts after crash tests.

"Generally all vehicles have some risk of fire in the event of a serious crash," NHTSA said in closing the two-month investigation.

No real-world fires were reported by regulators or the automaker, but the investigation has cast a shadow over GM's heavily promoted bid to lead on fuel efficiency and green technology with the Volt.

NHTSA opened its investigation after separate fires last year.

A battery pack in a Volt crash tested in May caught fire three weeks later at a NHTSA facility in Wisconsin. In lab tests completed in late November, a second Volt pack began to smoke and throw off sparks, while a third battery pack caught fire a week after a simulated crash.

In response to the safety concerns, GM has said it will strengthen structural protection for the 400-pound lithium-ion battery in the Volt, and take other steps to prevent coolant fluid from leaking and triggering a fire.

The automaker said in a statement on Friday that NHTSA's decision is consistent with the results of its own analysis

With the government investigation closed, GM will avoid a formal recall and related public distractions as it seeks to improve on disappointing 2011 Volt sales, which totaled about 8,000.

GM will service those vehicles plus another 6,500 that were produced through last December 31, NHTSA said. Free repairs for consumers should begin in February under a customer satisfaction plan.

All vehicles manufactured this year will have the fix, NHTSA said. GM would not disclose the cost of the repairs.

NHTSA ran the same side-impact test performed last May on the newest model Volt, which was equipped with the modifications, and no battery damage, coolant leak or fire was reported, the agency said.

The government ended its work ahead of a hearing next week in the House of Representatives on Volt fires. GM Chief Executive Dan Akerson is expected to testify as are regulators.

Lawmakers are interested in why GM and NHTSA said nothing publicly about the initial fire until months after it occurred.

(Reporting By John Crawley; Editing by Gary Hill and Bob Burgdorfer)

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