AU
B178A

DTC B178A indicates the Stage 2 Passenger Front Airbag inflator circuit resistance measures 0 ohms (or near 0 ohms; normal range is 2 — Atto 3

Safety System

DTC B178A indicates the Stage 2 Passenger Front Airbag inflator circuit resistance measures 0 ohms (or near 0 ohms; normal range is 2.0–3.0 ohms). "Stage 2" refers to the secondary trigger circuit of a dual-stage airbag, providing greater deployment energy during a severe collision.

A resistance of 0 ohms indicates a hard short (short to ground or short to power), not an open circuit.

This fault triggers the SRS fail-safe mechanism: the control unit disables the passenger airbag Stage 2 deployment function, or disables the airbag entirely, and illuminates the instrument cluster SRS warning light.

During a collision, this causes insufficient passenger protection, posing a serious safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Airbag inflator internal short circuit: A manufacturing defect, long-term aging, moisture ingress, or previous accident damage reduces the internal squib resistance to 0 ohms, creating a hard short circuit.
  • 2Wiring harness short to ground/power supply: Metal edges cut the internal dashboard wiring harness insulation, fixing screws pinch and chafe the insulation, or water ingress corrodes the harness, causing a short to vehicle body ground or +12V power supply.
  • 3Connector terminal fault: The dedicated yellow front passenger airbag connector (usually located behind the glovebox or inside the dashboard) has bent terminals, backed-out pins, loose connections, or electrolyte corrosion from water ingress causing a short circuit between pins.
  • 4SRS ECU internal detection circuit fault: A damaged internal analog-to-digital converter (A/D converter) or detection chip in the airbag control unit causes incorrect resistance readings.
  • 5Improper repair procedures: Connecting or disconnecting the airbag connector without disconnecting the battery, causing an arc, or using a standard multimeter resistance setting to directly measure a live airbag circuit, causing the internal squib to blow or short-circuit.
What To Do
  • 1
    Safety preparation: Switch the vehicle OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds (120 seconds for some models) to fully discharge the SRS backup capacitor. This prevents accidental airbag deployment and personal injury during repair.
  • 2
    Fault Status Confirmation: Connect the diagnostic tool and read the B178A freeze frame data. Confirm whether the fault is current (Active) or historical (History). Record the ambient temperature and vehicle status at the time of the fault. Check for accompanying SRS fault codes (such as B1780, B1781, etc.).
  • 3
    Preliminary visual inspection: Remove the glove box or front passenger lower trim panel. Inspect the SRS-specific yellow wiring harness and connectors for obvious damage, scorching, or water ingress. Verify the airbag identification label is intact and confirm it is a genuine part.
  • 4
    Circuit isolation diagnosis: Disconnect the front passenger second-stage airbag module connector and connect a special airbag substitute resistor (2.7Ω/0.25W) to the wiring harness side. Connect the diagnostic tool and read the data stream. If the tool displays a normal resistance (2.0-3.0Ω), the fault is in the airbag module. If it still displays 0Ω, the fault is in the wiring harness or the ECU.
  • 5
    Harness continuity check: Use a high-precision multimeter to measure the resistance to ground at both ends of the airbag harness (must be >1 MΩ), measure the continuity between both ends of the harness (must be <1 Ω), and check for a short circuit to the +B power supply (voltage must read 0 V with the ignition switch ON).
  • 6
    Airbag module inspection: If the wiring harness is normal, measure the resistance between the airbag module pins. Normal resistance is 2.0–3.0 Ω. A reading of 0 Ω or <1 Ω confirms an internal module short circuit. Replace the entire front passenger airbag assembly (do not dismantle or repair).
  • 7
    Repair and replacement: Repair the damaged wiring harness (use high-temperature insulating tape and corrugated conduit; maintain clearance from the vehicle body). Replace short-circuited connectors or the airbag module. Use only genuine parts for airbag replacement. On some models, use the diagnostic tool to perform the 'Airbag Replacement Record' or 'VIN Write' operation.
  • 8
    System Reset and Verification: Reconnect all connectors and reconnect the battery. Use the diagnostic tool to clear the fault code. Perform "SRS System Configuration" and "Acceleration Sensor Zero-Point Calibration". Perform an ignition cycle self-test (IG ON → IG OFF three times). Verify DTC B178A does not return and the SRS warning lamp turns off.
Real-World Cases
BYD DTC AI Analysis

Water-damaged vehicle, passenger-side A-pillar wiring harness chafed and shorted

✓
A 2019 BYD Song MAX developed active fault code B178A after driving through heavy floodwater. Inspection revealed standing water under the front passenger carpet that hadn't drained. The wiring harness retaining clip inside the A-pillar had detached, causing the harness to chafe against the metal bracket until the insulation wore through and rainwater caused a short to ground. Repair action: Cleaned up the water and dried the interior thoroughly, replaced the damaged wiring harness section (approx. 40cm), re-secured the harness routing with cable ties, and ensured more than 20mm clearance from metal edges.
BYD DTC AI Analysis

Aftermarket airbag module internally shorted after accident repair

✓
2021 BYD Tang. After front-end collision repairs at a non-authorized shop, the SRS light stayed on following passenger airbag replacement. DTC B178A stored and would not clear. Harness side resistance measured normal, but the replacement airbag module (aftermarket second-hand unit) showed 0.3Ω across the terminals—indicating an internal short in the gas generator. Installed a new genuine airbag module; resistance returned to 2.4Ω and the fault cleared. Note: Airbag components must be genuine new parts; second-hand parts present serious safety hazards.
BYD DTC AI Analysis

Blocked AC drain caused connector corrosion and short circuit

✓
BYD Yuan EV owner reported the SRS warning light occasionally coming on. Scan tool showed DTC B178A as an intermittent fault. Inspection found the air conditioning evaporator drain hose blocked with leaves; condensate overflowed behind the glove box and oxidized the passenger airbag connector pins (yellow 2-pin plug), creating verdigris and a micro-short. Cleaned the connector terminals with electronic contact cleaner, applied conductive protective grease, cleared the drain hose, and wrapped the connector with waterproof tape.
BYD DTC AI Analysis

Dashboard frame pinched wiring harness after aftermarket large screen installation

✓
A Qin Pro logged code B178A one week after its centre screen was replaced at a third-party modification workshop. Removing the dashboard revealed the installer had failed to properly reposition the passenger airbag wiring harness. The instrument panel metal frame crushed the harness, fracturing the internal copper conductors and causing a short to earth. Technicians rerouted the wiring, fitted rubber protective sleeving, and repaired the damaged conductors to resolve the fault. Have qualified professionals perform any dashboard removal work, and verify SRS wiring harness integrity.
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.