In November, NHTSA opened an investigation after two incidents in which the Volt's battery pack either caught fire or emitted sparks in the days or weeks following crash tests.
An earlier battery fire occurred in June, three weeks after the agency completed side-impact testing on the Volt.
In a statement, GM said NHTSA's decision to close the investigation "is consistent with the results of our internal testing and assessment." The automaker reiterated that the change it's making to protect the battery pack "is intended to make a safe vehicle even safer."
The agency's clearing of the Volt helps GM avert a potential hit to its image. The automaker's executives have held up the revolutionary car as a symbol of innovation and fresh thinking at the post-bankruptcy GM.
Despite broad praise for the car, U.S. sales of 7,671 Volts last year fell short of GM's goal of 10,000 units.
GM executives cooperated with NHTSA's investigation but have maintained that the Volt is safe. Company executives say the voluntary fix will make the car "safer" by reinforcing the steel surrounding the battery pack to prevent it from being punctured during a crash. It also will add a sensor to the battery pack to monitor coolant leaks.
GM is asking its 8,000 Volt customers to visit their Chevy dealership to have the work done. Dealers will be ready to perform the work starting in February, GM said.
GM said that about 250 Volt owners have taken GM up on its offer to provide loaner vehicles or to buy back the car to quell any safety concerns during the probe.
Testimony still to come
NHTSA said it still is unaware of any real-world Volt crashes that have resulted in a battery fire. It said the agency took the "unusual step" of opening the investigation because it wanted to "ensure the safety of the driving public with emerging [electric vehicle] technology."
A U.S. House panel hearing is scheduled on Wednesday to scrutinize how GM and regulators handled the investigation of the fire risks. GM CEO Dan Akerson has agreed to testify.
The House Oversight and Government Reform Committee, chaired by U.S. Rep. Darrell Issa, R-Calif., is also expected to hear from David Strickland, administrator of the National Highway Traffic Safety Administration.