What does the B0040 fault code mean?
The B0040 fault code, applicable to all car manufacturers, means “Third Row Left Side Airbag Deployment Control (Subfault)“.
Background Information: Side airbags are mainly divided into two categories: side torso airbags and side curtain airbags. Side torso airbags deploy from the seat or door panel to protect the occupant’s torso. Head protection is provided by an extension of the side torso airbag or by a head curtain airbag.
Possible Symptoms of B0040 Fault:
- SRS/Airbag warning light illuminates – steady or flashing after engine start.
- Third-row left passenger airbag malfunction – the vehicle may display secondary messages such as “Passenger airbag off”.
- Intermittent or continuous light – may only appear after a cold start, or remain continuously lit.
- No change in driving performance – engine performance, transmission shifting, and other non-SRS functions remain normal.
- These signs point directly to a problem with the third-row left side airbag deployment control module, not an engine or chassis component issue.
Possible causes of B0040 include:
- Short circuit in the left front/driver’s side deployment loop
- Damaged or faulty left front/driver’s side airbag module
- Damaged or faulty left front/driver’s side airbag
- Damaged or faulty wiring harness
- Poor electrical connection
- Water or corrosion in the airbag system
B0040 Fault Diagnosis and Repair
- Connect a dedicated SRS scan tool (e.g., an airbag scanner with airbag scanning capabilities). Read the B0040 definition and any associated freeze frame data. Confirm that the code is active, not pending or intermittent.
- Perform a bidirectional test on the third-row left side airbag circuit. The scanner should command the module to report sensor voltage and driver output status. Values outside of manufacturer specifications (sensor input is typically 0–5 V, driver resistance is 0 Ω to 10 Ω) confirm a hardware fault.
- Inspect the wiring harness between the airbag module and the SRS controller. Check for corrosion, wire wear, or damaged connectors. Repair or replace any damaged parts, then retest.
- Check the module power and ground with a multimeter. During ignition, the voltage should be stable at 12 volts (+/- 0.5 volts), and the ground resistance should be less than 0.1 Ω. Abnormal readings indicate a supply problem that must be resolved before further testing.
- Attempt to reprogram the module using the manufacturer’s flash procedure. If the OBD2 scanner tool reports successful reflashing and the fault is cleared, perform a road test and rescan to confirm that the fault code does not return.
- If communication or internal tests still fail, the module is likely irreparable. Replace it with a new control unit that matches the VIN and program it according to the vehicle’s SRS calibration data.
How to Avoid DTC B0040
You can take several steps to avoid DTC B0040:
- Avoid driving through deep water or puddles.
- Avoid spraying water directly into the engine compartment.
- Keep the engine compartment clean and free of debris.
- Regularly inspect the wiring harness for damage or corrosion.
- Have a qualified technician inspect and maintain the airbag system regularly.
The airbag system is a critical safety system in a vehicle. If the airbag system is not working properly, the airbags may not deploy in a collision, which could lead to serious injury or even death. If you see the airbag warning light on your dashboard, be sure to have a qualified technician diagnose and repair your vehicle as soon as possible.