Airbag warning light on (SRS): causes and what to do

The clock spring (also called a spiral cable or clock spring cassette) is a coiled ribbon of electrical conductor housed inside the steering column. It maintains a continuous electrical connection between the rotating steering wheel and the fixed wiring of the car for the driver's airbag, horn, and steering wheel controls, even as the wheel turns. Because the clock spring experiences constant flexing every time the steering wheel is turned, it is a wear item that fails over time, typically on vehicles with 80,000 miles or more. When the clock spring fails, the SRS control module loses contact with the driver's airbag and triggers the airbag warning light. A faulty clock spring may also cause the horn or steering wheel buttons (cruise control, audio, phone) to stop working, which is a helpful additional symptom.

Typical repair cost: Clock spring replacement: £120 to £350 including parts and labour on most vehicles. The part itself is typically £40 to £150; the labour involves removing the steering wheel, which requires the battery to be disconnected and an airbag depowering wait period.

Modern vehicles are fitted with seat occupancy sensors (also called seat mat sensors or occupant classification systems) in the front passenger seat. These sensors determine whether the seat is occupied by an adult, a child, a child seat, or is empty, and relay this information to the SRS control module so the passenger airbag deployment is calibrated appropriately. If the sensor fails or its readings fall outside expected parameters, the SRS module detects a fault and illuminates the airbag warning light. Seat occupancy sensor faults are among the most common causes of the airbag warning light on vehicles aged 5 years or more. They can also be triggered by heavy items placed on the passenger seat (such as a heavy bag or briefcase) that confuse the sensor into registering a weight that does not match a recognised occupant profile.

Typical repair cost: Seat occupancy sensor mat replacement: £120 to £350 including labour. On some vehicles the sensor is integral to the seat foam and the seat cushion must be replaced as a whole, increasing costs to £200 to £600. Recalibration or reset only (if the sensor has not failed but has been confused): £40 to £80.

Crash sensors (also called impact sensors or accelerometers) are positioned around the vehicle at strategic points, typically in the front bumper area, the B-pillars, and under the sill, to detect collision forces above a threshold that should trigger airbag deployment. If one of these sensors develops an internal fault, becomes corroded (particularly those mounted in wheel arch liners or near the sill where they are exposed to water and road salt), or loses its wiring connection, the SRS module cannot confirm a complete, functioning sensor network and triggers the warning light. On vehicles that have been involved in minor bumps, even impacts that did not trigger airbag deployment, impact sensor damage is worth considering as a cause of a subsequently appearing airbag light.

Typical repair cost: Individual crash sensor replacement: £80 to £250 including labour. Wiring repair to a damaged sensor: £60 to £200. Full sensor set replacement (if multiple are corroded on a high-mileage vehicle): £250 to £600.

The SRS (supplemental restraint system) control module is the central computer that monitors all airbag and seatbelt pretensioner circuits, stores crash event data, and triggers deployment when sensors indicate a collision above the threshold. If the module develops an internal fault, it can no longer guarantee that airbag deployment will occur correctly and illuminates the warning light. SRS module failures can be caused by water ingress (the module is often located under the centre console where a drink spill or water leak from the windscreen can reach it), electrical faults such as a short circuit, or degradation of internal components over time. On vehicles where airbags have previously deployed, the SRS module stores a crash code that cannot be cleared by a simple reset: the module must be replaced or professionally reprogrammed.

Typical repair cost: SRS module replacement and programming: £250 to £700 on most vehicles. SRS module repair by a specialist electronics firm (where the module is sent away for repair): £80 to £300, which is significantly cheaper than a new module. Crash data clearing and module reprogramming (where the module is not physically damaged): £60 to £150 at a specialist.

When an airbag deploys in a collision, the inflator inside it is a single-use pyrotechnic device. The deployed airbag fabric, the inflator, and the SRS module that recorded the crash event must all be replaced before the supplemental restraint system can function again in a subsequent collision. Vehicles that have been repaired after an accident and returned to the road without a full SRS system replacement will have a permanently illuminated airbag warning light and no functioning airbag protection for occupants. This situation occasionally arises with vehicles purchased at auction or through private sale where the cosmetic damage from a collision has been repaired without the underlying safety system restoration. It can also arise from partial repairs where only some of the deployed airbags were replaced.

Typical repair cost: Full SRS restoration after airbag deployment: £600 to £2,500 depending on how many airbags deployed and what other pretensioner or curtain components were triggered. This cost is typically covered by insurance in a properly processed claim.

The SRS control module monitors its supply voltage constantly, because an airbag system that deploys during a low-voltage condition (for example, a battery that cuts out in an accident) would be unreliable. If the battery voltage drops significantly below 12 volts, either due to a failing battery, a charging system fault, or an extended period with the ignition on and no engine running, the SRS module may log a voltage fault code and illuminate the airbag warning light. This is one of the more frustrating causes of the airbag warning light because it can mimic a genuine SRS component failure. Low battery voltage as a cause is particularly common after a battery has been jump-started incorrectly or after a new battery has been fitted without following the SRS reset procedure specified by some manufacturers.

Typical repair cost: Battery replacement (if the battery is the root cause): £80 to £200 including fitting. SRS fault code clear after battery issue is resolved: £40 to £80 at a garage. No SRS parts replacement should be needed if low voltage is the only cause.