(Adds NHTSA statement, paragraph 9)
WASHINGTON, April 7 (Reuters) - General Motors, which has recalled 2.6 million cars for faulty ignition switches that caused air bags to deactivate, may also have a defect in air bags in 2003 to 2010 Chevrolet Impalas, an auto safety watchdog group said on Monday.
In a letter to the National Highway Traffic Safety Administration, the Center for Auto Safety said that the computer code, or algorithm, which determines when an air bag deploys in an accident, may improperly turn off the air bag if the passenger is bounced in his or her seat just before an accident.
Air bags are not designed to deploy when a passenger's weight is below a certain amount, and a bouncing motion, even when the person is belted, could reduce the weight registered by the seat sensor linked to the air bag algorithm.
There have been 143 fatalities in front-impact crashes in 2000-2010 model year Impalas in which the air bags failed to deploy, according to the center, and in 98 of them, the occupants were wearing safety belts. It said it included the 2000-2002 models in the crash data because it was possible that some of them contained the suspect algorithm.
"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 center's executive director, said in the letter.
Ditlow credited accident investigator Don Friedman of Xprts LLC with uncovering the alleged defect. Friedman in November had called on NHTSA to open an investigation.
A NHTSA probe could ultimately lead to a recall.
"We will, of course, cooperate with NHTSA if it determines any further action is needed regarding this petition," GM spokesman Greg Martin said.
NHTSA said in a statement it "has received and is evaluating a petition about possible defects" in the deployment of air bags in certain Impalas.
The air bag algorithm has also become part of the investigation into a recall begun in February of 2.6 million Chevrolet Cobalts, Saturn Ions and other GM models for a defective ignition switch that could suddenly turn off the car's engine.
Questions have been raised about whether the largest U.S. automaker was trying to conceal the problem, after years of consumer complaints and at least 13 deaths linked to the faulty part.
NHTSA acting Administrator David Friedman told a congressional hearing last week that his agency's investigators were surprised to learn that turning off the engine caused the air bags to deactivate immediately in the recalled GM cars.