OBD-II trouble code
B0040: Third Row Left Side Airbag Deployment Control (Subfault)
A fault in the firing loop of a torso airbag built into the left third-row seat. Because this bag lives inside the seat rather than the body structure, its wiring is the only airbag circuit in the vehicle that has to survive being folded flat every time someone needs the cargo space.
Quick facts
- System
- Body
- Category
- Airbag / SRS Restraints
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $1,200
- DIY difficulty
- Shop recommended
What does B0040 mean?
Everything unusual about this code follows from one fact: the airbag it describes is bolted into a seat, and seats move.
A side airbag of this type is a torso bag. It sits inside the outboard bolster of the seat back, and in a side impact it inflates in the narrow space between the occupant's ribcage and the trim panel. That is a different device from the curtain, which drops from the roof rail to cover the glass, and different again from a frontal bag. The distinction matters when ordering parts and when reading a scan tool, because a vehicle can have all three on the same side and each has its own loop and its own code.
Being seat-mounted is what shapes the failure pattern. Every other pyrotechnic circuit in a vehicle runs through structure that never changes shape. This one runs down through the seat frame and out to a connector at the seat base, and on a third-row bench that assembly gets folded flat, tumbled forward, and in some vehicles powered down into a well in the floor. The harness is built with a service loop and a routing clip precisely so it can tolerate that, and the two most common faults are the two things that defeat it: a clip that was never refitted after trim or upholstery work, so the loop now snags on the frame, and a connector at the seat base that has been walked out of its lock by repeated cycling.
There is a second hazard specific to seat-mounted airbags that is worth stating plainly, because it is common and preventable. Aftermarket seat covers are not compatible with a seat that contains a side airbag unless they are specifically made and certified for it. A generic cover does two bad things at once: it puts a layer of unbroken fabric across the seam the bag is designed to burst through, which can redirect or restrain the deployment, and fitting it usually means pulling and tucking around the frame in exactly the area the airbag harness runs. A B0040 that appeared shortly after new covers went on is very likely to have a physical explanation somewhere under them.
Equipment scope is the last thing to settle. Third-row side airbags are fitted to some large three-row SUVs and vans and not to others, and a code can be present in a module's table without the hardware being installed on that particular build. Confirm from build data before diagnosing a loop that may not exist.
This fault does not affect how the vehicle drives. It affects whether a rear occupant gets side-impact protection, and the circuit contains a live pyrotechnic device, so the diagnosis belongs with a technician who can power the system down safely.
Common causes
- Seat-base connector walked out of its lock by repeated folding and tumbling of the third-row seat
- Harness routing clip missing after upholstery, trim or child-seat anchor work, letting the loop snag on the seat frame
- Aftermarket seat cover fitted over a seat-mounted airbag, disturbing or pinching the loop during installation
- Chafed conductor where the harness crosses the seat frame at a fold point
- Failed inflator squib in the seat-mounted side airbag module
- Yellow SRS connector damaged or its shorting bar bent during a seat removal
- Water ingress at the seat base on vehicles with a leaking rear hatch seal or tail-light gasket
- Restraints control module fault, which should be concluded only after the seat harness is proven
Symptoms
- SRS or airbag warning lamp illuminated and staying on
- B0040 stored in the restraints module with a symptom byte describing open, short or resistance out of range
- Fault that sets or clears in step with folding, tumbling or raising the third-row seat
- Code that first appeared within days of seat cover installation or interior trim work
- No change whatsoever in engine operation, braking, steering or fuel economy
- Occasional single-trip appearance after heavy cargo loading over the folded seat
- Other restraints functions still operating normally, since each loop is independent
- Nothing visible in the seat itself, as the bag and its wiring are entirely concealed
Diagnostic steps
- 1.Ask what has been done to the seat recently. On a seat-mounted airbag loop, upholstery work, seat covers, child-seat anchor installation and cargo barrier fitting are all far more likely explanations than a spontaneous inflator failure, and the answer usually points at a specific spot to inspect.
- 2.Confirm from build data that this vehicle actually has a third-row side airbag. Modules carry code tables covering options the individual vehicle may not have, and diagnosing a loop that was never installed wastes a full diagnostic hour.
- 3.Power the system down following the manufacturer's procedure and wait the specified capacitor discharge interval before touching any connector. This step is not optional on a circuit containing a live pyrotechnic device.
- 4.Cycle the seat through its full range — fold, tumble, raise — with the loop monitored, then inspect the harness at each fold point. Seat-mounted loops fail mechanically far more often than electrically, so reproducing the movement is more informative than a static measurement.
- 5.Inspect the seat-base connector for a partially released lock and for terminals pushed back in the housing. Repeated seat cycling works connectors loose gradually, and a plug that looks seated can still be a millimetre short of contact.
- 6.Only after the harness and connector are proven, measure loop resistance against the manufacturer's specification with the approved tool to decide between wiring and a failed inflator. Never bridge or apply power to a deployment loop with an ordinary meter.
Repair cost
$100 – $1,200
Refitting a dislodged connector or replacing a missing routing clip may cost only the diagnostic time, and it is a realistic outcome on a seat-mounted loop. SRS diagnosis is typically $110 to $210. Repairing or replacing a damaged section of seat harness runs $180 to $500 because the seat generally has to come out. A seat-mounted side airbag module is the expensive end at $400 to $1,000 fitted, and note that it is usually sold as an assembly with part of the seat back trim rather than as a bare inflator. Restraints module replacement with programming adds several hundred dollars more.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with airbag control module (sdm) replacement / reset preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.