OBD-II trouble code
B0040: Third Row Left Side Airbag Deployment Control (Subfault)
The airbag/SRS control module detected an electrical fault in the third-row left side airbag deployment loop. That rear side airbag may not deploy as designed, so this needs prompt professional attention.
Quick facts
- System
- Body
- Category
- Airbag / SRS Restraints
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does B0040 mean?
B0040 is a body (B) code stored by the airbag control module. Its SAE-generic definition is 'Third Row Left Side Airbag Deployment Control,' referring to the firing loop that deploys the side airbag protecting the left-side third-row seating position in a side impact. It is the third-row counterpart of B0030 (Second Row Left Side Airbag) and B0020 (Left Side Airbag) at the front. Only vehicles with a third row and third-row side airbags — some larger three-row SUVs and vans — have this loop, so confirm the exact application against your make's service data before assuming a third-row side airbag is fitted.
The module sets the code when the deployment loop's resistance falls outside the tight window these pyrotechnic circuits require — an open circuit, a short to power or ground, or resistance that is too high or too low. A symptom byte appended to the code identifies the exact fault type. A third-row side airbag is usually mounted in the seat or the adjacent trim, so its firing loop runs through a connector back near the rear of the cabin that is easily disturbed by folding the third-row seats, loading cargo, or trim service. That connector, and any harness disturbed during seat or cargo-area work, are the most common trouble spots.
This is a supplemental restraint fault, not a driveability fault: the vehicle drives normally, but the third-row left side airbag may not deploy as intended in a crash, and airbag circuits carry a small risk of unintended deployment when mishandled. SRS diagnosis and repair should be performed by a qualified technician who can safely disable the system, and the fault should not be left unrepaired.
Common causes
- Corroded, backed-out, or loose connector near the third-row left seat
- Damaged wiring in the rear side-airbag loop after seat folding, removal, or cargo damage
- Failed third-row side airbag inflator (squib)
- Fault in the SRS connector or its shorting bar at the airbag
- Prior side-impact damage or repair that disturbed the loop
- Faulty airbag/SRS control module
Symptoms
- Airbag / SRS warning light on
- Stored B0040 fault (often with a symptom byte) in the restraints module
- No effect on engine or driving performance
- Fault that may appear or clear as the third-row seat is folded or moved
Diagnostic steps
- 1.Use a scan tool that can access the airbag/SRS module and record B0040 with its full symptom byte and any companion codes.
- 2.Confirm the vehicle actually has a third-row side airbag and locate its connector for the specific make and model.
- 3.With the system safely disabled per service procedure, inspect the rear connector and the seat-frame harness for corrosion, backed-out terminals, or crush damage.
- 4.Fold and adjust the third-row seat while watching for an intermittent fault, since these loops are commonly disturbed by seat movement or cargo loading.
- 5.Measure the deployment-loop resistance against specification to separate a wiring/connector fault from a failed inflator.
- 6.If the seat or airbag was recently removed or replaced, verify the connector is fully seated and the correct part was installed.
- 7.Repair the wiring/connector fault or replace the indicated component, then clear codes and confirm the SRS light stays off.
Repair cost
$100 – $900
Reseating or repairing the rear connector can be inexpensive, while a failed seat-mounted airbag or control module is much costlier. SRS diagnostic time typically runs $100-$200, and a third-row side airbag or SDM/RCM replacement with programming can reach several hundred dollars or more. SRS work should be done by a qualified technician.
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.