Airbag System Overview: How the SRS Works

Written by Anthony Calhoun, ASE Master Tech A1-A8

Airbag System Overview: SRS Components, Operation, and Technician Safety

If you work on modern vehicles long enough, you will eventually find yourself elbow-deep in a steering column, pulling a dash, or dealing with a collision repair, and the airbag system will be right there in your way. Understanding how Supplemental Restraint Systems work is not optional for a competent technician. It is a matter of doing the job right and not sending a customer out the door with a safety-critical system in unknown condition. It is also, frankly, a matter of not blowing an airbag in your face while you have your head under the dash.

This article covers the full SRS picture: what it is, how it works, what all the parts do, and how to work around it safely. No shortcuts. No vague warnings. Just the technical information you need to do the job correctly.

What the SRS Actually Is

SRS stands for Supplemental Restraint System. That word supplemental is not an accident. Airbags are designed to work in addition to seatbelts, not instead of them. Every airbag system in production assumes the occupant is wearing a seatbelt. When a belted occupant is in a frontal crash, the seatbelt holds them back while the airbag inflates into the space in front of them and cushions the deceleration. The airbag is a controlled explosion that happens to save your life when everything goes as designed.

An unbelted occupant is a different scenario entirely. An unbelted driver will be moving toward the steering wheel at the same speed as the vehicle at the moment of impact. The airbag deploys in approximately 25 to 50 milliseconds. If that driver is already moving toward the wheel, the airbag may deploy while their face is at the wheel, and the inflating bag becomes the thing that causes the injury rather than prevents it. This is why airbag-related fatalities happen almost exclusively in unbelted occupants, children out of position, and out-of-position adults who are leaning forward at the time of the crash.

When a customer comes in and says their airbag light is on, the correct response is to diagnose it, document it, and fix it. An inoperative SRS system is a safety defect, full stop. You should not return a vehicle to a customer with a known SRS fault and no documentation of the conversation.

SRS Components: What Is in the System

A modern SRS system has several interconnected components. You need to know what each one does before you can diagnose a fault or safely work around the system.

Airbag Control Module (ACM / SDM / RCM)

The ACM goes by different names depending on the manufacturer. General Motors calls it the Sensing and Diagnostic Module (SDM). Ford calls it the Restraints Control Module (RCM). The function is the same regardless of the name. The ACM is the brain of the system. It monitors all crash sensor inputs continuously, runs a crash pulse algorithm to determine whether deployment is warranted, and fires the squib circuits that trigger airbag inflators and seatbelt pretensioners.

The ACM also contains its own internal accelerometer. This means the module itself is a crash sensor in addition to being the controller. It stores fault codes, deployment event data, and in many applications, the deployment data is locked in after a crash and the module must be replaced rather than reused. Some ACMs can be reset and reused after a minor fault event, but any module that has recorded a deployment is typically a one-time unit.

The ACM has a power reserve capacitor built in. This capacitor stores enough energy to deploy airbags for a short window of time after the vehicle's main power supply is cut. This is why disconnecting the battery alone does not immediately make the system safe. The wait time before touching the system is not a suggestion.

Crash Sensors

Crash sensors come in several forms. Front impact sensors are typically located at the front of the vehicle, often at the radiator support or within the front rail structure. Side impact sensors are located in the B-pillars, doors, or seat structures. Rear impact sensors, where present, are mounted at the rear of the vehicle and are used to trigger head restraint systems or rear seat airbags.

Satellite sensors are remote accelerometers that feed signal data back to the ACM. They are not standalone controllers. The ACM interprets the signal from multiple sensors simultaneously to determine the crash direction, severity, and appropriate deployment response. A frontal crash may fire front airbags only. A side impact may fire side curtain and side torso bags on the impact side only. The system is calibrated to deploy the right bags for the specific crash event, and it does this in milliseconds.

Occupant Classification System (OCS)

The front passenger seat on most modern vehicles includes an Occupant Classification System. The OCS uses a weight mat or bladder system built into the seat cushion to detect occupant presence and size. If the system classifies the front passenger seat as empty or occupied by a small child under a weight threshold, it will suppress or reduce the deployment of the front passenger airbag. This is important for technicians to know because any seat work on a front passenger seat may involve OCS components. Incorrectly reassembled seat cushions can cause a fault code and potentially suppress the passenger bag permanently until it is fixed.

Seatbelt Pretensioners

Pretensioners are explosive devices built into the seatbelt assembly, typically at the retractor end. When a crash event meets the deployment threshold, the ACM fires the pretensioner squib, which drives a small piston that retracts the seatbelt by several inches in milliseconds. This takes up slack in the belt before the occupant can load it, positioning the occupant properly to interact with the deploying airbag. Pretensioners are one-time-use devices. After any deployment event, every pretensioner that fired must be replaced. They look intact after firing, and the belt still functions as a regular retractor, but the pretensioner charge is spent.

Airbag Inflators

The inflator is what generates the gas that fills the bag. Historically, most inflators used sodium azide as the propellant. Sodium azide produces nitrogen gas when ignited, which is what inflates the bag. It is highly toxic in its raw form, but the deployment process converts it entirely to nitrogen, which is inert. Modern inflators have largely moved away from sodium azide to alternative propellants and in some cases to cold gas generator designs that use compressed argon or a similar gas stored under pressure. The Takata recall is directly related to inflator chemistry, which is covered in its own section below.

How Deployment Works

When a crash occurs, the ACM receives input from crash sensors in the form of deceleration data. The ACM runs this data through a crash pulse algorithm, which evaluates the severity and direction of the crash in real time. Not every hard stop triggers deployment. The system is calibrated to distinguish between a curb hit, a severe pothole, and an actual crash severe enough to require airbag deployment.

When the algorithm determines deployment is warranted, the ACM fires the squib circuit. The squib is essentially a small initiator, similar to a blasting cap. It carries current from the ACM through two wires to the inflator. The squib ignites the propellant, gas is generated, and the airbag inflates from a folded module into its full volume. The entire process from crash detection to full inflation happens in approximately 25 to 50 milliseconds. The bag then vents immediately through vent holes, deflating within a fraction of a second so the occupant can exit the vehicle. The powder residue left in the cabin after deployment is a combination of cornstarch used as a bag lubricant and combustion byproducts. It is not toxic in normal exposure conditions but should be ventilated.

Types of Airbags

Modern vehicles carry a range of airbag types depending on the vehicle platform and trim level.

• Frontal driver airbag: Mounted in the steering wheel hub. Deploys forward toward the driver's face and chest in frontal impacts.
• Frontal passenger airbag: Mounted in the instrument panel above the glove box. Deploys on the same crash signal as the driver bag in most frontal events.
• Side curtain airbags (SAB/CAB): Run along the roofline from A-pillar to C-pillar or further. Deploy downward to cover the side glass and protect the head in a side impact or rollover. Many are designed to stay inflated for several seconds in rollover events.
• Side torso airbags: Mounted in the seat back of front seats, sometimes rear seats. Protect the chest and abdomen in a side impact.
• Knee airbags: Mounted below the instrument panel on the driver side, sometimes passenger side. Protect the knee and lower leg and help manage forward body movement in a frontal crash.
• Rear seat airbags: Found in some luxury vehicles. Protect rear seat occupants in frontal or rear impacts.
• Pedestrian hood airbags: Found on select vehicles such as some Volvo and Lexus models. Deploy over the hood and windshield base to cushion impact with a pedestrian detected by external sensors.

Knowing what bags are on the vehicle you are working on matters. A dash R&R on a base trim vehicle may have a different bag count than the same model in a higher trim. Always verify with the OEM service information before starting any SRS-adjacent work.

The Clock Spring

The clock spring, also called a spiral cable or coil assembly, is a rotating electrical connector that lives inside the steering column between the column and the steering wheel. Its job is to maintain continuous electrical contact with the components mounted in the wheel, including the driver airbag, horn, cruise control switches, audio controls, and anything else wired into the wheel, while still allowing the wheel to rotate freely in both directions.

Internally, the clock spring is a flat ribbon cable wound in a spiral. As the wheel turns left or right, the ribbon winds or unwinds within the housing. The housing limits how far the wheel can rotate before the ribbon reaches its travel limit, which is why centering the clock spring before installation is critical. An off-center clock spring will run out of ribbon travel at full lock in one direction, breaking the ribbon and taking out everything the clock spring feeds.

Clock springs fail with age, heat cycling, and heavy use. When they fail, you will typically see an airbag fault code along with loss of horn function, loss of cruise control, or loss of wheel-mounted audio controls. The airbag fault alone with no other symptoms can also point to a clock spring, but when you have horn and cruise issues at the same time as an airbag light, the clock spring is near the top of your list.

Replacement procedure requires disconnecting the battery and waiting the appropriate time before removing the wheel. The wheel must be marked or confirmed in the straight-ahead position before removal. The new clock spring must be centered before installation according to the OEM procedure, which typically involves a locking tab or a counting procedure to find center position. Install the wheel, reconnect battery, clear codes, and verify with a scan tool that SRS communication is restored and no faults remain.

SRS Safety Procedures for Technicians

This section is not filler. Every item here represents a way you can hurt yourself or damage a system if you skip it.

Battery Disconnect and Wait Time

Before working on any SRS component, disconnect the negative battery terminal. The wait time before touching SRS components varies by OEM. Most manufacturers specify a minimum of one to two minutes. Some OEMs, including certain Honda and Toyota applications, specify up to ten minutes. This wait time is for the backup capacitor in the ACM to discharge. Do not skip this or estimate. Pull the OEM service information for the exact vehicle and follow the specified wait time.

Never Use a Test Light on Squib Circuits

A test light draws enough current to fire a squib. If you probe an airbag squib circuit with a standard test light, you may deploy the airbag while you are holding it. This is not a hypothetical scenario. It has happened. The squib circuit wiring on most vehicles is yellow, and SRS connectors typically have a yellow housing or yellow secondary lock to identify them. Do not probe them with a test light, and do not use a standard ohmmeter on a live squib circuit.

Resistance Measurement on Squib Circuits

If you need to measure squib resistance for diagnostic purposes, use a dedicated SRS squib resistance tool that limits current output to a level that cannot fire the squib. Some scan tools have this function built in. Measuring resistance on a live squib circuit with a standard DVOM is not safe practice regardless of the meter's output spec, because you do not know the squib's exact firing threshold and the meter's internal resistance may not protect against it. Disconnect the module from the circuit and use the proper tool.

Airbag Storage and Handling

Removed airbag modules must be stored face-up, meaning the deployment surface faces up. If an airbag were to accidentally deploy while face-down, the force would launch the module like a projectile. Keep removed airbag modules away from heat sources and away from static discharge. Do not toss them in a parts bin with other components. Treat them as what they are: live explosive devices that happen to also be safety equipment.

Static Discharge Precautions

Static discharge is a legitimate concern when handling squib circuits. In dry shop conditions, a static discharge from your body can theoretically fire a squib. Ground yourself before handling squib connectors. Many OEMs supply shorting clips or shunt plugs that short the squib terminals when the connector is disconnected from the module or the bag. Use them. Do not remove the shunt until you are ready to make the final connection.

Reading SRS Fault Codes

SRS faults are B-codes, meaning body codes in the OBD standard. A code like B0001 might indicate a frontal crash sensor fault, while B0051 might indicate a driver airbag squib open circuit, depending on the manufacturer's code definitions. B-codes are not universal the way P0xxx powertrain codes are. You need OEM service information or a manufacturer-specific scan tool to interpret them accurately. A generic scan tool code reader may pull the code number but will often give you an inaccurate description.

The airbag warning light behavior tells you something before you even plug in a scanner. A light that comes on at key-on and goes out after a few seconds is the normal bulb check. A light that stays on after the check indicates an active or stored fault. A light that flashes a specific number of times on some older vehicles is flashing a code in the same manner as OBD-I engine codes. A light that never comes on at all means the bulb is out or the circuit feeding the light has a problem, which is itself a fault worth diagnosing.

Once you pull codes, address the root cause before clearing and retesting. An airbag light that comes back on after clearing without finding a cause means the condition that set it is still present. Work through the diagnostic tree in the service information rather than trying to clear codes and hope the light stays off.

The Takata Recall: Why Technicians Must Check Every Vehicle

The Takata airbag inflator recall is the largest automotive recall in history, covering tens of millions of vehicles across dozens of manufacturers. The issue is with Takata-manufactured inflators that used ammonium nitrate as the propellant without a drying agent. Over time, exposure to heat cycling and humidity causes the ammonium nitrate to degrade and recrystallize. When a degraded inflator fires, it can rupture the metal housing and send metal shrapnel into the vehicle occupant at high velocity. Multiple deaths and hundreds of injuries have been attributed to Takata inflators. This is not a minor quality issue. It is a demonstrated fatal defect.

As a technician, you have an obligation to check every vehicle that comes into your shop for open recalls, and the Takata recall in particular. The NHTSA recall lookup at nhtsa.gov takes a VIN and returns any open recalls in seconds. If a vehicle has an open Takata recall, inform the customer, document the conversation, and follow your shop's process for recall notification. Many manufacturers have dealer-only remedies for recall repairs, but making sure the customer knows is your responsibility regardless.

The recall also affects how you handle parts. If you are working on a vehicle with a recalled Takata inflator and you need to remove the steering wheel for another repair, do not reinstall a known-defective inflator and return the vehicle without addressing the recall. Document it, inform the customer, and make sure the shop has a process in place.

Working Around Airbags During Other Repairs

Much of the time you interact with the SRS system, it will be incidental to another repair. Steering column work, dash removal, and seat removal all require you to work around airbag components.

For steering column work and wheel removal, the procedure is always the same: disconnect battery, wait OEM-specified time, remove the steering wheel using an appropriate puller, handle the airbag module carefully, and store it face-up. Never use an impact wrench on the steering wheel center bolt if the airbag is still in place. The vibration is not a safe way to work around a live explosive device.

Dash removal on most modern vehicles requires disconnecting multiple SRS connectors including the passenger airbag, knee airbag if equipped, and any SRS wiring routed through the dash structure. Yellow connectors with shunt plugs. Disconnect in the right order per service information. Do not yank connectors or pull on the wiring.

Seat removal requires attention to side torso airbag connectors and OCS wiring. The OCS connectors are especially sensitive to improper handling. Always reconnect OCS wiring before reassembling the seat, clear codes after the job, and verify the passenger airbag status light in the instrument cluster is reading correctly for occupied and unoccupied seat states.

Post-Collision Inspection: What Must Be Replaced

After a vehicle has been in a collision that deployed any airbag, the job is more than just replacing the bags. Every SRS component that deployed is a one-time-use device and must be replaced. But the list goes further than that.

Non-deployed airbags on the opposite side of the vehicle from the crash may or may not need replacement depending on the OEM's position. Some manufacturers require replacement of all airbags after any deployment event. Others allow retention of non-fired bags. Pull the OEM collision repair documentation for the specific vehicle and follow it exactly. This is not a place to guess or make a judgment call based on what looks okay.

Any crash sensor that took a physical hit, regardless of whether it fired, should be replaced. The sensor housing may appear intact while the internal accelerometer has been damaged or shifted out of calibration. A damaged crash sensor that reads incorrectly is worse than a missing one because it may prevent deployment in the next crash or cause an inadvertent deployment under the wrong conditions.

Final Word

Airbag systems are not something to approach casually. They contain explosive devices, operate on high-current firing circuits, and protect the people sitting in the vehicle from death. A poorly executed repair or an improperly diagnosed fault leaves someone exposed to exactly the risk the system was designed to eliminate.

Every time you work near an SRS component, follow the procedure. Disconnect the battery. Wait the time. Use the right tools. Handle the components correctly. And before you hand the keys back to the customer, verify that the system is fully functional and documented. That is not extra work. That is the job.