AU
B178A1A

DTC B178A1A indicates the SRS control module detects the Passenger Second Stage Front Airbag igniter circuit resistance at or near 0 ohms — Atto 3

Safety System

DTC B178A1A indicates the SRS control module detects the Passenger Second Stage Front Airbag igniter circuit resistance at or near 0 ohms.

The second-stage airbag uses dual-stage ignition technology to control the deployment time interval based on collision severity.

A resistance of 0 ohms indicates a short circuit fault in the airbag igniter circuit (short between wires or short to ground), not an open circuit.

This fault causes the SRS system to enter degraded mode: the system disables this specific airbag circuit to prevent accidental deployment and illuminates the airbag warning light.

In a frontal collision, the passenger side may lose second-stage cushioning protection, deploying only the first-stage airbag or failing to deploy entirely, which significantly reduces occupant protection.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Front passenger side airbag module internal igniter short circuit: Fused airbag internal igniter bridge wire or damp pyrotechnic charge, causing a sudden drop in resistance.
  • 2Dashboard internal wiring harness chafed and shorted: Vibration and friction damage the Stage 2 airbag wiring harness insulation (usually located on the dashboard crossmember or behind the glovebox), causing a short to body metal.
  • 3Connector water ingress and corrosion: A/C condensate leakage or vehicle wading causes a short circuit between pins of the front passenger-side floor wiring harness connector (such as CJ103 or near the G301 ground point).
  • 4SRS control module internal detection circuit fault: A damaged ECU internal sampling resistor or A/D conversion circuit triggers a false short circuit fault.
  • 5Modification or repair damage: Screws crush, drill bits pierce, or retaining clips pinch the wiring harness when installing a dashcam or audio system on the front passenger side, or during accident repairs.
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds to allow the SRS backup power supply to fully discharge.
  • 2
    Fault confirmation: Use a VDS or Launch diagnostic tool to read all DTCs. Confirm B178A1A is a current (Active) fault, not a historical fault. Check for accompanying fault codes (such as B178B - front passenger second-stage resistance too low).
  • 3
    Physical inspection: Remove the front passenger dashboard lower trim panel. Visually inspect the yellow SRS wiring harness corrugated conduit for damage or burn marks. Check the airbag connector (usually located behind the glovebox or on the dashboard crossmember) for looseness or water ingress.
  • 4
    Airbag module inspection: Disconnect the front passenger second-stage airbag connector (use a shorting clip or anti-static measures). Measure the resistance between the airbag-side terminals using a digital multimeter. The normal value is 2.0-5.0 Ω. If the reading is 0-1 Ω, replace the front passenger airbag module assembly.
  • 5
    Harness continuity check: Measure the resistance to ground of the circuit between the SRS ECU-side harness connector (ECU disconnected) and the airbag connector. The normal value is infinite. If the reading is 0 Ω or a low resistance, locate the short circuit in the harness. Focus on the dashboard bracket pass-through hole and chafing points between the floor harness and the body.
  • 6
    ECU verification: If the wiring harness and airbag are normal, measure the resistance between the corresponding pins at the SRS ECU connector. If the resistance remains 0 Ω, the ECU has an internal fault. Replace the airbag control module.
  • 7
    Repair and Verification: After repairing or replacing the faulty component, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code, perform the 'SRS system self-check' and 'configuration write' (if replacing the ECU), and verify the fault code does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV Passenger Airbag Short Circuit After Wading

✓
A 2019 BYD Qin EV presented with a constant airbag warning light after driving through deep floodwater. Scanned DTC B178A1A. Removed the interior trim and found the passenger-side floor wiring harness connector CJ103 had accumulated condensation due to a blocked air conditioning drain tube, causing internal pin oxidation and shorting. Cleaned the connector and applied conductive adhesive, replaced the wiring harness seal, and cleared the fault. Also check the air conditioning drain tube for blockages.
BYD DTC AI Analysis

BYD E2 short circuit from pinched wiring harness after accident repair

✓
2020 E2 set DTC B178A1A after front collision repair. Inspection found the instrument panel crossmember mounting bolts installed during the accident repair were too long, penetrating the passenger side second-stage airbag harness corrugated tubing and causing the wire cores to short to body. Replaced the damaged harness section (Part No.: BYD-BCM-SRS-05) and adjusted the bolt length. Fault resolved. Note: Accident repairs must strictly follow repair manual torque specifications.
BYD DTC AI Analysis

BYD E3 airbag module internal igniter aging

✓
An E3 with 80,000 km exhibited intermittent B178A1A faults in humid conditions. With the harness disconnected, the airbag module resistance read normal at ambient temperature but dropped sharply to 0.5 Ω when heated to 60°C. Diagnosed as thermal breakdown of the aged internal igniter insulation. Replaced the passenger airbag assembly and dashboard trim panel (integrated design). Fault resolved.
BYD DTC AI Analysis

Aftermarket passenger-side glove box modification damaged the wiring harness

✓
The owner fitted ambient lighting to the passenger glovebox, drilling holes that damaged the second-stage airbag wiring harness. DTC B178A1A appeared intermittently. Inspection revealed the drill bit had scratched the wiring behind the glovebox, exposing the copper core which contacted the metal dashboard bracket. Cut out the damaged section, soldered in an extension wire, and insulated with double-layer heat shrink tubing to repair the harness. Restored the wiring to its original routing position. Fault cleared. Recommend prohibiting non-professional modifications involving SRS circuits.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.