If you’ve ever set a bag of groceries or something of a similar weight on your car’s front passenger seat and it caused an “airbag off” light to illuminate, it means your vehicle has determined that whatever is riding shotgun does not warrant a frontal airbag. More specifically, sensors in the passenger-seat cushion have sent feedback to a control unit as part of what’s known as an Occupant Classification System (OCS). It’s a device that detects the presence or absence of someone or something in the front passenger seat, and it’s found in most newer, U.S.-sold cars and trucks.
You can thank the government, which starting in 2003 began requiring advanced airbags in all light passenger vehicles. That means that they can inflate with less energy or not deploy at all, based on the size and weight of the occupant of the passenger seat. The reasoning behind this mandate is to save kids or small adult passengers from being injured or killed by the front airbag, which bursts open in about 1/20 of a second.
Children under age 12 are not supposed to ride in the front seat of any car or truck. It is universally recommended by automakers and regulators that they ride in the rear seat, where there’s less chance of injury. But for instances in which you must have a small person—whether child or adult—up front, the OCS exists.
While federal law requires cars and trucks to have the systems, it does not include a common weight threshold for deactivating the airbag. Instead, each automaker’s OCS must meet safety performance criteria for crash-test dummies representing a 12-month-old infant, a three-year-old toddler, a six-year-old child, and a 5th-percentile woman, which is a person with a height of 64 inches and a weight of 108 pounds. How this is all done is up to each manufacturer.
“At one time there were all sorts of different schemes out there,” said Guy Nusholtz, a senior manager in the experimental and computational mechanics department at Fiat Chrysler. Pressure sensors, cameras, and lidar have all been used to detect vehicle occupants. Now, most of the OCSs are composed of “load cells” to detect “local forces” on the seat, Nusholtz said. “But what you have to do is detect not only the weight, but the way the dummy sits in the seat,” he said. “You have to accommodate a spectrum of different seating situations.”
Using data from the sensors in the seat, algorithms take into account the weight of the occupant, where the person is sitting in the seat, and whether or not a seatbelt is buckled. Even though there is no across-the-board weight threshold at which point the passenger airbags turn off, C/D asked a few automakers at what point their systems decide to turn off the passenger airbag.
Natalie Kumaratne of Honda environment and safety public relations told us the “airbag off” indicator comes on in its cars when weight sensors detect 65 pounds or less on the seat. A Toyota spokesman said the company has a “magic number” but declined to tell us what that is, saying it is “something that we would not disclose.” And as FCA’s Nusholtz said, a certain weight threshold is only one of several considerations.
The guide to knowing when your “airbag off” light might light up is often found in the vehicle’s owner’s manual. Take this chart found in the literature that comes with the 2016 Mazda CX-9, for example:
And Ford is even more succinct in its owner’s manual for the 2017 Escape:
So, if the “airbag off” light in your car is glowing and you’re not sure why, the best place to start is the owner’s manual. If you’re still not sure what’s going on with the light, it’s probably a good idea to take it to a mechanic. In general, you can remain worry-free by not having a child or small person ride in the front seat, if a back seat is available. Better still, avoid crashes that result in airbag deployment.
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