In May, GM recalled 42,904 2012 and 2013 model Buick LaCrosse and Regal cars. U.S. safety regulators have opened an investigation of some recalled General Motors Co. sedans after a vehicle fire in March led them to question a screening test’s effectiveness.

The U.S. National Highway Traffic Safety Administration said it has opened what it calls a recall query to look at whether a stress test GM is using on a portion of the recalled cars is good enough, because the fire occurred after the procedure that was meant to catch the problem.

In May, GM recalled 42,904 2012 and 2013 model Buick LaCrosse and Regal cars, and 2013 model Chevrolet Malibu Eco cars equipped with its “eAssist” mild hybrid system to repair circuit boards that may overheat and lead to a loss of battery charge or, in extreme cases, a fire in the trunk.

Overheating of the circuit boards in the generator control module in some of the vehicles may cause problems, including loss of battery charge and the illumination of a malfunction indicator light. The issue does not involve the eAssist battery.

If the warnings are ignored, the engine may stall. There may also be a burning or melting odor.

Of the recalled cars, about 22,000 are getting their battery packs replaced, while the rest undergo the screening test to see if the control module needs to be replaced, GM said. A spokesman said the recall is continuing and the automaker is cooperating with NHTSA in its probe.

The spokesman said GM remains unaware of any injuries or crashes related to the issue and he pointed out that changes in the production process at the plant were made and no other vehicles are affected.

NHTSA questioned the effectiveness of the screening test because the fire in March occurred in a car that had already gone through the testing as part of a service procedure before the recall. That same test is being used on almost half the cars in the recall.

Regulators said in documents filed online that the fire drew “into question whether or not the procedure can effectively identify a defective (control module).”

The recall query could lead NHTSA to ask GM to replace the control module in the recalled cars that underwent the stress test.