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I have a new -week-old Ford Focus with miles on...

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Dear Tom and Ray:



I have a new (5-week-old) Ford Focus with 1,630 miles on it. My 17-year-old son was driving it (alone) at about 35 mph, and he decelerated quickly because he thought he missed his turn. He is adamant that he didn't hit anything, and I examined the car with a fine-tooth comb and found not a mark on it. However, both the driver and passenger air bags deployed! The passenger air bag shattered the windshield on that side, even forcing it outward in a bubble shape. When I called the dealer, the guy said he had never heard of an air bag going off without the car hitting something. The car was towed in, and I am waiting for the mechanic to tell me what the "crash sensor" readings are. I'd like to believe my son. Is it possible that the air bags deployed without him hitting anything? -- Chris

TOM: Having recently suffered with a 17-year-old boy in my house, my inclination is to consider him guilty of everything. But I'm quite sure your son is innocent here, Chris.

RAY: In order for the air bags to deploy as designed, the car would have to hit something while going at least 15-20 mph. And if it had hit something at that speed, you certainly wouldn't need a fine-tooth comb to see the damage. The front of the car would look like Lawrence Welk's accordion.

TOM: My only guess is that the deployment was caused by a defect in the car's air-bag system. This car has a single, centralized sensor that triggers the air bag. It's a micro-electronic device (called a capacitive accelerometer) that measures the speed of deceleration. Obviously, when you hit something -- a bridge abutment or another car -- you decelerate much faster than you EVER could using the brakes.

RAY: The signal from that accelerometer is then sent through a computer, which decides whether the air bags should be deployed.

TOM: So perhaps the accelerometer itself is faulty (although there are two of them that run in parallel to guard against the failure of one of them), or perhaps the computer that processes the information is faulty. But in either case, the air bags should not have deployed based on emergency braking alone.

RAY: So unless your dealer has another convincing argument, I'd insist that he replace the entire air-bag-control module, including the sensor and computer.

TOM: You should also report this incident to the National Highway Traffic Safety Administration (NHTSA), which operates an Auto Safety Hotline (888-327-4236). You can report stuff to them by e-mail from our Web site -- the Car Talk section of www.cars.com --too. While we don't keep track of how many Ford Focuses have had accidental air-bag deployments, NHTSA does. And if they get enough reports about a certain make and model, they'll open an investigation, which could lead to a recall.

RAY: As for the kid, there are plenty of other things we can find him guilty of. He says he "decelerated quickly when he thought he missed his turn." Right. He was probably trying to see if the radio volume would go past "10" and then stomped on the brakes when he saw a 17-year-old girl walking by. So tell him to pay closer attention to the road and not to drive so fast. But I don't think he's responsible for the air bags, Chris. I suspect Ford is.

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