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air bags deployed after crash

About Air Bags & Crash Sensors

by Larry Carley copyright

Air bags have been in the news in recent years because of deaths that have resulted from air bag deployments in relatively minor low speed crashes. The victims have been small children or infants in the passenger seat, or small female adults drivers who were too close to the air bag or unbelted when it deployed. The deaths, in some cases, have been blamed on improper use of infant seats or not using seat belts. But others blame the deaths on government regulations that require auto makers to use air bags that deploy with sufficient force to protect an unbelted 160 lb. male adult in a 30-mph crash. This requires deployment speeds of up to 200 mph, which can cause serious injury or even death to children and small adults who are not buckled up or are too close to the bag when it deploys. In Europe, where rules allow air bags that deploy with 30% less force, there have been no deaths attributed to air bags.

The National Highway Traffic & Safety Administration (NHTSA) has reviewed their regulations and now allows auto makers to use less powerful air bags. NHTSA now allows auto makers to add switches for deactivating the air bags, and even allows consumers to have a switch installed (by a professional) to deactivate their air bags under certain circumstances. Auto makers have also developed smarter "adaptive" air bag systems that can vary their deployment speed and force depending on crash circumstances. Such systems modify air bag deployment force based on occupant size, seating position and impact speed.

For more information, see NHTSA's website information on Air Bags & air bag on-off switches

In recent years there have been many recalls for airbag related problems. For more information on this subject see Airbag Recalls.

Regardless of what type of air bag system a vehicle has, though, all motorists are urged to use their seat belts, to sit as far back as possible from the steering wheel, and to put infants and children 12 and under in the back seat.

air bag deployment illustration
Air bags have to deploy within milliseconds when a crash is detected.


One thing all air bag systems share in common are crash sensors. In theory, an air bag is only supposed to deploy in a frontal collision that is severe enough to trigger the control electronics, or in some cases a mechanical firing pin.

Most car companies say a vehicle has to experience an impact that generates at least 7 G's of force before it will trigger the air bag. For most vehicles, this would be the equivalent of hitting a solid barrier at 12 to 15 mph, or hitting another vehicle at a speed of about 25 mph. In reality, the air bag may deploy at higher or lower speeds.

Air bags also are not supposed to go off in rollovers or side or rear collisions, but they sometimes do. Weird things can happen during an accident, and impact forces can be transmitted in such a way that the air bag sensors are tricked into thinking a frontal collision is occurring when in fact it really is not. Of course, in such situations the air bag does nothing to protect the occupant because it is only designed to protect during frontal collisions.


An air bag is only as good as its control system. On most vehicles, the bag is triggered electrically. Located in the front of most domestic vehicles are one to three "crash" sensors. These are positioned well forward in the crush zones so they will react almost instantly to the sudden deceleration that results from a frontal impact (anything up to about 30 degrees either side of center). Many European vehicles use only a single electronic crash sensor located inside the passenger compartment. The same setup is now being used in many newer domestic vehicles.

There are several different types of crash sensors. A commonly used sensor in older vehicle is the electromechanical "gas dampened ball and tube" design. The sensor is nothing more than a small tube with a switch at one end and a gold plated steel ball at the other, held in place by a small magnet. When the sensor receives a hard enough jolt to knock the ball loose from the magnet, the ball rolls down the tube, hits the switch and closes the circuit. The tube is slanted upward so the ball should return to its original position after an impact.

Another common crash sensor in older vehicles is the "Rolamite" design by TRW. Inside is a small metal roller that rolls forward under sudden deceleration and trips a switch.

Some vehicles have "spring and mass" crash sensors in which a spring loaded weight is deflected by the impact to close a switch. Most newer vehicles now have solid state crash sensors that contain either a piezoelectric crystal or a "micromachined accelerometer" chip that produces an electronic signal when jolted.

Because many crash sensors will automatically reset themselves after a hard jolt, some car makers say their crash sensors do not have to be replaced after an accident as long as the sensors have not suffered damage. These include Acura, Audi, Chrysler, Ford, Infinity, Lexus, Mazda, Nissan and Toyota. The problem, of course, is knowing for sure whether or not a crash sensor has been damaged or has failed to reset itself.

To prevent false deployments that might result from bumping into objects or a slow speed fender bender, most air bag systems also have one or two "safety" or "arming" sensors which are usually located inside the passenger compartment (under the dash or seat, in the airbag control module, or in the steering wheel air bag module). The safety sensor will not allow the bag to deploy unless it also experiences a certain rate of deceleration (usually less than that of the crash sensors).

If both the crash sensors and safety sensor(s) are triggered by a collision, then and only then does the electronic control module say okay and ignite the air bag inflator. It does this by applying voltage to the "squib" inside the sealed inflator. The squib sets off the sodium azide pellets inside the inflator, which produce harmless nitrogen gas as they explode. The gas rushes through a filter that traps any unburned particles of propellant and helps cool the gas, and forces the bag to burst from its housing and unfurl in 30 to 55 milliseconds (less time than it takes to blink your eyes).

airbag module
Airbag control module


A few older vehicles (Jaguar & Toyota) are equipped with a self-contained "mechanical" air bag system, manufactured by Breed Automotive (a similar system is also available from Breed as an aftermarket add-on driver side air bag system). The mechanical air bag system does not have electromechanical or electronic crash sensors. It uses a mechanical trigger to ignite the propellant that inflates the air bag. It works something like a land mine. Inside the air bag assembly is a single mechanical impact sensor that trips a firing pin when a severe enough jolt is experienced. The firing pin ignites a primer which sets off the sodium azide pellets to inflate the bag. The mechanical system eliminates the need for a power supply as well as any control electronics or external crash sensors, which makes it much easier (and cheaper) to replace after an accident.


The air bag control module self-checks the crash sensors every time the engine is started, so unless the air bag warning light is on the sensors are assumed to be okay. If a fault is detected, the air bag warning light will come on and usually deactivate the air bag system. Using a scan tool, you can pull the trouble code from the system and refer to the appropriate diagnostic chart in a service manual to troubleshoot the problem. Air bag service information can also be found on the vehicle manufacturer's website (Click Here for a list of websites and access fees). Loss of circuit continuity anywhere in the air bag system, or loss of power to the air bag module are common causes of trouble codes.

Because crash sensors are sealed units, you cannot always determine their true condition by outward appearances. Any sensor that is obviously sustained physical damage as a result of a collision or other damage should be replaced. But what about ones that look okay? Most electromechanical crash sensors are designed to be electrically open in their rest condition. So one quick check you can perform is to check for continuity with an ohmmeter. If the sensor contacts are closed, it has not reset and should be replaced.

CAUTION! Do not attempt to check or replace any crash sensor unless the air bag module has first been deactivated (or deployed as a result of an accident). This can be done by unplugging the air bag connector at the base of the steering column and waiting at least 10 minutes or longer depending on the application (always refer to a service manual for the proper deactivation and removal procedure).

Also, do not use a self-powered test light or jumper wires on the wiring of a live air bag system. Apply voltage to the wrong circuit and you could accidentally trigger the bag!

Testing a electromechanical crash sensor in a vehicle that has been in an accident to see if the sensor is electrically open, however, does not necessarily mean the sensor is okay because the sensor may have sustained internal damage from the force of the collision that may prevent it from working properly. Because of this, other car makers say crash sensors should always be replaced if an air bag has been deployed in an accident. These include BMW, General Motors, Isuzu, Mercedes, Mitsubishi, Porsche, Saab, Volkswagen and Volvo.


Several very important points must be kept in mind about replacing crash sensors. One is to make sure the replacement is the correct one for the application. Crash sensors are calibrated for specific vehicle applications, so compare OEM part numbers to make sure your parts supplier has given you the correct replacement crash sensor.

WARNING! The car battery must be disconnected before you attempt to replaced a crash sensor or work on any part of the airbag system. This is to reduce the risk of accidentally triggering the air bag while you are working on the vehicle. Wait at least 20 minutes after disconnecting the battery to start your repairs. This will give the stored electrical energy in the airbag system capacitors time to discharge. NOTE: Some European makes may take even longer to fully discharge, so refer to the service literature for specifics.

The mounting of the crash sensor is also critical. A replacement sensor must be installed in exactly the same location and the same position as the original. The sensor must also be firmly attached so it won't break loose in a future collision. Altering the mounting location or position of a sensor may cause it to trigger the air bag accidentally or not at all.

air bag


If your vehicle has been involved in an accident, it may be necessary to replace the air bag module in addition to the airbags and/or crash sensors. The module on many late model vehicles records hard codes and crash data at the moment of impact. This information cannot be cleared with a scan tool, so the dealer will tell you the module also has to be replaced when your vehicle is repaired. Car makers also want the module replaced to minimize their liability in case your vehicle is involved in another accident.

If you are attempting to replace your own air bag module, always disconnect the battery cable, wait at least 20 minutes, then locate the module (usually somewhere under the dash) and disconnect it from the wiring harness. Do not reconnect the battery cable until after the new air bag module has been installed and connected to its wiring harness.


The airbag control module in most late model vehicles also functions like a flight recorder "Black Box" and stores important data when an airbag deploys during an accident. Important factors that may have a bearing on why the accident occurred such as vehicle speed, vehicle direction (if the vehicle has a GPS navigation system), whether or not the brakes were being applied, and the status of the occupants seat belts (fastened or unfastened) are all recorded in the black box memory.

This information can be very useful for investigating and reconstructing an accident, and for proving (or disproving) any negligence on the part of the driver.

The NHTSA (Part 563 regulation) requires all 2014 and newer cars and light trucks to be equipped with a black box data recorder. Recorders have been installed on many vehicles for the past two decades, and for model year 2013 nearly 95 percent of new vehicles were compliant with the NHTSA rules. The law requires uniform methods for collecting, storing and retrieving onboard motor-vehicle crash data, which includes crash speed, braking, accelerator pedal, safety belt and airbag deployment information.

The question is, who has access the black box information?

A number of states have passed laws that restrict access to this information, depending on circumstances. Generally speaking, the information is usually private and belongs to the vehicle owner. It cannot be accessed without the vehicle owner's permission. However, if driver negligence is suspected, or the information is deemed crucial for an accident investigation, authorized law enforcement personnel may have the right to download and review it.

Federal law prohibits removing, modifying or tampering with the black box recorder or altering any of the data that is stored within it.

Some Historical Facts About Airbags

Historical Fact: September 2010

New Airbag Law to Protect Consumers Against Airbag Fraud

Rhode Island recently approved a new law to combat airbag fraud. The law outlines actions that would constitute 'deceptive trade practices,' such as:

Failing to replace any deployed airbags when a vehicle undergoes collision repairs following an accident, including installing any object in lieu of an airbag, including any light manipulating system that fools the airbag light and causes it to remain off.

Selling or installing any device that defeats an airbag system, renders the system inoperative, or prevents the system from turning on the airbag warning light if the system is defective or inoperative.

Indicating a vehicle has an operating and functional airbag system when in fact it does not due to a previous accident or failure to make required repairs.

The Rhode Island law includes fines of $1000 to $2000, and up to two years in prison for violations, and finds of up to $100,000 and up to 10 years in prison is serious injury results from a violation.

Historical Fact: October 10, 2012

NHTSA Warns Consumers About Counterfeit Air Bags

If you have had an air bag replaced in the past three years, the NHTSA says it might be counterfeit. Tens of thousands of counterfeit air bags that do not inflate or fail to inflate properly have reportedly been installed in many vehicles. In at least one case, a counterfeit bag fired shards of metal shrapnel on impact, the agency said.

You can go to for information on how to contact a call center established by auto manufacturers to learn if your vehicle model is among those for which counterfeit air bags are known to have been made. However, it is not clear how these counterfeit air bags can be easily identified by police or accident investigators. No deaths or injuries have been tied to the counterfeit bags according to the NHTSA.

The announcement by NHTSA concerning counterfeit airbags underscores the importance of checking the history of used cars. Airbags are one of most crucial parts of a car's safety system. To help consumers, Carfax has created a free service to check for prior airbag deployments that have been reported. Go to

In addition, the airbag systems of any used car, especially those with prior damage, should be inspected by a qualified mechanic, body shop or airbag specialist to ensure they are working properly. This further highlights the need to document quality repairs, especially those that may impact a vehicles airbags and other safety systems.

Historical Fact: May 2015

CONSUMER ALERT! Massive Takata Airbag Recall Affects 34 Million Vehicles

Airbags made by Takata Corporation are being recalled after it was discovered that the inflator shells in some airbags can shatter and explode if the airbag is deployed. Metal shrapnel from the inflator can pass through the airbag and injure or kill vehicle occupants. Five people have reportedly died as a result of such injuries, and others have lost eyes or have suffered severe facial disfigurement or cuts from the metal shrapnel.

The massive airbag recall affects 34 million vehicles made from 2002 to 2014 by numerous vehicle manufacturers, including Acura, BMW, Chrysler, Dodge, Ford, General Motors, Honda, Infiniti, Lexus, Mazda, Mitsubishi, Nissan, Subaru and Toyota. Consumers are urged to check with their new car dealer to see if their vehicle has a defective airbag. If your vehicle is included in the recall, you are urged to contact your dealer to schedule an airbag replacement as soon as possible. There is no cost to replace airbags covered under the recall.

Motorists can also check to see if their vehicle is included in the recall by going to the website and entering their Vehicle Identification Number (VIN number). The VIN number can be found on a metal plate at the base of the windshield on the driver's side of your car or truck. The VIN number is also listed on your vehicle registration and auto insurance card. If your vehicle is involved in the recall, contact your dealer ASAP to schedule an airbag replacement.

Some auto makers say deactivating your airbags until replacements are available is NOT recommended. However, other car makers have deactivated airbags in some vehicles at the consumer's request as a preventative measure. The airbag system can be deactivated by locating and removing the airbag fuse. Once deactivated, the airbags will NOT deploy in an accident.

As of 2021, Takata is still recalling millions of airbags in many different makes and models of vehicles. See the link to Airbag Recalls below for updated information.

air bags More Safety Articles:

Airbag Recalls

Vehicle Crash Test Safety Ratings

Auto Accidents: Mechanical Problems That Increase Your Risk

Milestones in Automotive Safety

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