What is the SRS system in a car?
What is the SRS system in a car?
SRS stands for "Supplemental Restraint System" – a supplementary restraint system. It includes all passive safety components in the vehicle that provide protection in addition to the seat belt during an accident: airbags, belt tensioners, crash sensors, and the central airbag control unit.
The SRS does not replace the seat belt, but supplements it. The belt and SRS work together as a system – the belt holds the occupant in position, while airbags and belt tensioners cushion and distribute the impact energy. Only when both systems function correctly is the best possible protection ensured.
Which components are part of the SRS?
The SRS system consists of the airbag control unit as the central unit, several crash sensors, the airbag modules (front, side, curtain, knee), belt tensioners with load limiters, the seat occupancy mat, and the clock spring in the steering wheel. All components communicate with each other via the CAN bus.
| Component | Function | Position in Vehicle |
|---|---|---|
| Airbag Control Unit | Central control, deployment decision | Under center console or tunnel |
| Crash Sensors | Detection of impact and deceleration | Front, sides, B-pillars |
| Front Airbags | Protection in frontal impact | Steering wheel and dashboard |
| Side Airbags | Protection in side impact | In seat backrests or door panels |
| Curtain Airbags | Head protection in side and rollover | In headliner on sides |
| Knee Airbags | Protection of legs from steering column | Under dashboard |
| Belt Tensioners | Immediate tightening of belt in crash | On B-pillar / belt buckle |
| Seat Occupancy Mat | Detection of whether passenger seat is occupied | In seat cushion passenger side |
| Clock Spring | Electrical connection steering wheel–control unit | Behind the steering wheel |
How does the SRS system work?
The SRS control unit continuously monitors all sensors in the vehicle. During an impact, the crash sensors measure the deceleration and send this data to the control unit. Within milliseconds, the control unit decides which airbags and belt tensioners are deployed – coordinated with the type of accident, impact severity, and seating position.
Sequence During an Accident
- Impact detected: Crash sensors measure deceleration (within 5-10 milliseconds)
- Plausibility check: Control unit compares data from multiple sensors for confirmation
- Deployment decision: Type and severity of impact determine which restraints are activated
- Belt tensioners fire: Belts are immediately tightened to hold occupants in position
- Airbags fire: Gas generators fill the airbags within 30 milliseconds
- Energy dissipation: Airbags release air in a controlled manner to absorb impact energy
The entire system reacts faster than the blink of an eye. From impact to fully inflated airbag takes only about 30-50 milliseconds.
What does the SRS warning light mean?
The SRS warning light in the instrument cluster indicates the status of the restraint system. Brief illumination after starting (2-5 seconds) is normal – the system is performing a self-test. If the light stays on continuously, there is a fault and the SRS is completely or partially deactivated.
- Brief illumination at start: Normal – self-test successful
- Continuously on: Fault in system – airbags may not deploy
- Flashing: Active fault – visit workshop immediately
- Does not light up at all: Light defective or tampered with – also a fault
A continuously illuminated SRS light is a significant defect during MOT testing. The vehicle will not pass the main inspection as long as the fault exists.
What happens with an SRS defect?
In the event of a defect in the SRS system, the control unit deactivates the entire restraint system or parts of it for safety reasons. In an emergency, airbags and belt tensioners will not deploy – the occupants are only protected by the seat belt in an accident, without the supplementary protective effect of the SRS.
The control unit permanently stores the fault code in memory. Even if the cause was only a temporary loose connection, the fault remains stored and must be cleared with a diagnostic device. A professional diagnosis at the workshop clarifies the exact cause.
Common SRS Fault Causes
- Crash data stored: After an accident, the control unit blocks the system
- Seat occupancy mat defective: Passenger airbag is permanently deactivated
- Crash sensor corroded: Moisture damages the sensor electronics
- Clock spring broken: Driver airbag can no longer be deployed
- Connector oxidized: Contact problems at plug connections under the seats
Frequently Asked Questions About the SRS System
Here you will find answers to the most important questions about the SRS restraint system in cars.
Can you drive without a functioning SRS?
The vehicle is technically drivable, but in the event of an accident, airbags and belt tensioners provide no protection. In addition, the vehicle will fail the MOT test. We recommend having SRS faults repaired immediately.
Can the SRS system be repaired?
Yes. In most cases, the problem can be resolved by repairing the control unit, clearing crash data, or replacing individual components. Airbag24 repairs SRS control units for all brands – MOT-compliant and with over 20 years of experience.
What does an SRS repair cost?
The costs depend on the cause. A control unit repair at Airbag24 is up to 50% cheaper than a new part. Exact prices for your vehicle model can be found in our shop – we are happy to help you.
Is SRS and airbag the same thing?
Not quite. The airbag is a part of the SRS system. The SRS additionally includes belt tensioners, crash sensors, seat occupancy mats, and the central control unit. All components work together to provide optimal protection in an accident.