DETROIT – General Motors is preparing to adjust Chevrolet Volts in dealerships and on the Detroit-Hamtramck assembly line to protect their batteries from catching fire long after a crash, in a solution tentatively endorsed by the government’s crash-testing and recall agency.

The changes will strengthen the safety structure around the battery pack, preventing the vehicle from puncturing the case in severe crashes, GM product chief Mary Barra said Thursday in a conference call. In addition, GM will add a sensor to monitor battery coolant levels and a bracket to the coolant reservoir to prevent leakage.

A Volt battery case was punctured during a government crash test this past spring, causing coolant to leak, come in contact with electronics and then catch fire three weeks later. The National Highway Traffic Safety Administration in November simulated the effects of the crash test in three Volt batteries, and one caught fire after sitting around for a week. NHTSA then opened an official investigation into the extended-range electric car, which runs on battery power for 35 miles before using a gasoline generator for unlimited range.

Barra said the in-dealership fix, which should be ready by February, is not a true recall, because the company made its decision to fix customers’ cars without a government directive. NHTSA said it has not concluded its investigation into the Volt. But a December government crash test of a Chevy Volt with the strengthened battery pack did not cause either intrusion into the battery case or coolant leakage — both of which must be present to enable a post-crash fire, NHTSA said.

“As a precaution, NHTSA has monitored the crashed vehicle since the test and will continue to do so for one more week,” the agency said in a statement. “However, the preliminary results of the crash test indicate the remedy proposed by General Motors today should address the issue.”

When the Volt fire news surfaced this fall, GM was already on track to miss its target of 10,000 Volt sales this year. U.S. customers bought 7,671 this year. Between July and December, interest in buying a Volt dropped in half among general consumers, according to a market survey by CNW Research. But Chevrolet sold 1,529 Volts in December, by far the best sales month for the car since it was introduced a year earlier.