If an occupant is unbelted or very small, or the car is traveling very slowly, the air bag may not deploy because it could cause even more severe injuries. Depending on the angle, the side air bags may deploy but not the front ones. If a car is parked and turned off when it's hit, the air bags won't work.
GM's switches created an unusual problem. Because of insufficient resistance, they moved from the "run" position into the "accessory" or "off" position while the car was moving, possibly due to a bump from the driver's knee or the weight of a key chain. With the switch in that position, the engine stalled and the power steering and power brakes stopped working, making the car harder to control.
In a 2006 crash in Wisconsin, a Chevrolet Cobalt traveling at 71 mph suddenly stalled. Two seconds later — outside GM's 150-millisecond window — it hit a clump of trees. The ignition was found in the "accessory" position and the air bags didn't deploy. Two passengers died and the driver was severely injured.
GM says the air bags in newer cars would work for a slightly longer period of time if the ignition is off, but still less than a second.
If the engine had stalled while the ignition was still in the "run" position, the crash might have had a different outcome. In that situation, the air bags, steering, brakes and most other equipment would have had power for up to several hours depending on the amount of charge in the car's battery, GM said.
A report on the crash, completed by a team from Indiana University that was hired by the government, said the air bags may not have deployed because the ignition moved out of position. But the report also noted two other reasons: The trees bent when the car hit them, so the impact may not have triggered the air bags. And none of the occupants was wearing a seat belt. Air bags are meant to supplement seat belts, not to replace them.