General Motors, which has recalled 2.6 million cars for faulty ignition switches, may also have a defect in airbags in 2003 to 2010 Chevrolet Impalas, an auto safety watchdog group says.

The Center for Auto Safety (CAS) has written to the U.S. National Highway Traffic Safety Administration (NHTSA) calling for a probe into why airbags don’t deploy on the affected Impalas.

The NHTSA has said it will consider an investigation, according to Reuters.

The CAS says the airbags may turn off if a passenger is bounced in his or her seat ahead  of an accident.

Airbags are designed not to deploy if a passenger’s weight is below a certain level and the safety group says a bounce could convince the computer code related to the airbag that a lighter person is in the seat.

In its letter to the NHTSA, the safety group alleges there have been 143 fatalities in front-impact crashes in 2000-2010 model year Impalas in which the airbags failed to deploy.

Call for investigation

"We call on NHTSA to examine each of the fatal non-deployment crashes to determine whether the air bag should have deployed and why it didn't," Clarence Ditlow, the CAS's executive director, said in the letter.

It cited a government petition by a former GM researcher who said he found a software fault that can misread a passenger’s weight and lead to frontal airbags not functioning.

The petition also raised questions about the software for all airbags used in the auto industry as several manufacturers have had recalls over airbags that don’t deploy.

The NHTSA is already investigating how GM airbags deploy in the case of the 2.6 million vehicles being recalled for an ignition switch problem.

The recall of Chevrolet Cobalts, Saturn Ions and other GM models is now under way because of an ignition switch that suddenly turns off the car's engine. Turning off the car may have the effect of causing the airbags to deactivate, NHTSA acting administrator David Friedman told a congressional hearing last week.

Friedman said the NHTSA is also asking other automakers for information about how airbags deploy with a loss of power and how airbag software works.

Questions have been raised about whether General Motors, the largest U.S. automaker, had tried to conceal the ignition switch proble. Internal documents released last week before Congress appeared to show concern about the switches up to a decade before the recall.