AU
B178E

DTC B178E indicates a short to the vehicle power supply (B+, 12V) in the front passenger frontal airbag Stage 2 squib circuit — Atto 3

Safety System

DTC B178E indicates a short to the vehicle power supply (B+, 12V) in the front passenger frontal airbag Stage 2 squib circuit.

Modern BYD models use a dual-stage airbag design: Stage 1 provides low-power inflation for moderate collisions; Stage 2 provides high-power inflation for severe collisions.

The SRS ECU internal diagnostic circuit continuously monitors the airbag squib circuit resistance (normally 2-3 Ω) and its insulation to ground and power.

The ECU sets B178E when it detects a short to power in the Stage 2 squib circuit (resistance below the specified threshold, typically <200 Ω to power).

This fault forces the airbag circuit into a fail-safe state.

In extreme cases, the short circuit could accidentally deploy the airbag during driving (despite shorting bar protection) or prevent the designed staged inflation during a collision, risking occupant injury.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Front passenger airbag wiring harness worn or pinched: Retaining clips or metal edges cut the internal dashboard wiring harness during removal and installation, shorting the ignition wire to the constant power wire.
  • 2SRS ECU internal drive circuit fault: Airbag control module internal ignition drive transistor (IGBT/MOSFET) breakdown shorts the second-stage control terminal to power.
  • 3Front passenger airbag connector terminal issue: The connector mounts behind the glovebox or at the instrument panel frame. Backed-out pins, bent terminals, or corrosion from water ingress cause a short circuit to an adjacent power supply terminal.
  • 4Wiring error after accident repair: Incorrect routing of the wiring harness inside the dashboard after collision repairs causes the airbag wiring harness to tangle and short-circuit with the instrument panel lighting power wire or the air conditioning control power wire.
  • 5Airbag module internal short circuit: The Stage 2 squib inside the front passenger airbag assembly has an internal short circuit. This fault is rare but unrepairable; replace the airbag assembly.
What To Do
  • 1
    Safe power-off and wait time: Turn off the ignition switch, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS ECU internal energy storage capacitor and prevent accidental airbag deployment during repair.
  • 2
    Fault code freeze frame analysis: Use the BYD dedicated diagnostic tool (VDS2000/VDS3000) to read the freeze frame data for DTC B178E. Record the vehicle speed, timestamp, and ambient temperature at the time of the fault to determine if the fault is intermittent.
  • 3
    Visual inspection and connector check: Remove the passenger-side instrument panel lower trim and check the SRS wiring harness for obvious wear or burn marks. Disconnect the passenger airbag connector (usually yellow) and check the terminals for backed-out pins, corrosion, or foreign matter.
  • 4
    Wiring insulation test: Use a multimeter on the resistance setting to measure the resistance between the second-stage ignition circuit and the power supply (B+) at the ECU side. The normal reading is infinity (OL). If the multimeter shows continuity, trace the wiring harness to locate the short circuit. Focus on the dashboard frame pass-through holes and wiring harness mounting points.
  • 5
    Airbag module isolation test: Disconnect the airbag module and connect a dedicated airbag simulator (SRS Simulator) to the wiring harness connector. Clear and re-read the fault codes. If the fault code disappears, the airbag module is faulty. If the fault remains, the wiring harness or ECU is faulty.
  • 6
    ECU-side voltage check: Reconnect the battery. Leave the airbag module disconnected. Turn the ignition switch to ON. Use an oscilloscope or multimeter to measure the second-stage ignition circuit voltage to ground. Normal voltage is close to 0V (<1V). A 12V reading confirms a short to power. Repair the wiring harness.
  • 7
    Repair and Verification: Repair the damaged wiring harness (use heat shrink tubing or replace the entire harness). Maintain sufficient clearance (>50mm) between the wiring harness and the power cable. Reconnect all components, clear the DTC, and perform an SRS system self-check (execute 'System Configuration' or 'Coding' using the diagnostic tool). Finally, perform a vehicle crash simulation test (use the 'Actuation Test' function on the diagnostic tool to verify circuit continuity only, without deploying the airbags).
Real-World Cases
BYD DTC AI Analysis

Song MAX aftermarket dashboard modification caused wiring harness chafing and short circuit

✓
A 2019 Song MAX developed a dashboard rattle after the owner installed an aftermarket center touchscreen. While removing the dashboard mounting screws, the owner overlooked the harness routing and cut the passenger airbag second-stage harness on the sharp edge of the dashboard frame. The exposed copper contacted the dashboard illumination power wire (constant power). The airbag warning light stayed on with DTC B178E. Removed the dashboard assembly and found the yellow SRS harness insulation damaged, with the two internal conductors touching the red power wire. Replaced the complete harness from the SRS ECU to the passenger airbag (Part No.: BYD-SRS-HRN-02), rerouted the wiring properly using a dedicated harness protection sleeve, and cleared the fault.
BYD DTC AI Analysis

Tang DM: Internal SRS ECU short after water wading

✓
2021 Tang DM. Vehicle waded through water exceeding the door sill height; two days later the airbag warning light illuminated. Scan tool retrieved DTCs B178E and B178D (first-level short circuit). Inspection revealed water traces beneath the front passenger carpet at the SRS ECU mounting location. Technical analysis: SRS ECU seal failed, allowing water into the internal circuit board and causing a short between the second-stage ignition drive circuit and the power bus. The ECU is a sealed, non-repairable component, so we replaced the SRS control unit (Part No.: ECU-SRS-TANG-21), thoroughly removed the accumulated water from the cabin, and dried the wiring harness connectors. Fault resolved. On water-damaged vehicles, check the SRS ECU compartment sealing carefully.
BYD DTC AI Analysis

Yuan EV airbag connector pin backed out causing short circuit

✓
2019 BYD Yuan EV. Airbag warning light illuminated the day after cabin air filter replacement at the dealership (glove box removal required). Diagnosis revealed active fault code B178E. Investigation found the passenger airbag connector (mounted on the frame behind the glove box) was strained during removal and refitting. The second stage igniter circuit terminal (typically pins 3-4) backed out of the connector housing and contacted the adjacent instrument panel illumination power terminal. Repair: Used a dedicated terminal tool to reseat the backed-out pin, refitted the connector locking tab, and confirmed the terminal fully engaged. Cleared fault codes; repeated vibration testing of the wiring harness did not reproduce the fault.
BYD DTC AI Analysis

Qin Pro wiring harness polarity reversed after accident repair

✓
2020 Qin petrol. After front collision repairs, the airbag warning light stayed on. Scanned DTCs B178E (short to power) and B1781 (circuit resistance too high). Checked repair records: the front passenger airbag had deployed in the accident. The technician replaced the passenger airbag assembly but misconnected the wiring harness. Yuan EV and Qin models use dual-stage airbags with two connectors (or one 8-pin). The technician swapped the stage 1 and stage 2 plugs, ignoring polarity, and connected the stage 2 circuit to the stage 1 power monitoring line. Fix: Reconnected the plugs per wiring diagram CIR-SRS-QIN-2020. Light out.
BYD DTC AI Analysis

Tang EV SRS ECU software false positive fault

✓
2021 BYD Tang EV. No collision or modification history. Intermittent airbag warning light. DTC B178E (intermittent). Circuit testing found no obvious shorts; wiggling the harness did not reproduce the fault. Further diagnosis revealed outdated SRS ECU software (V1.2) with a known EMI sensitivity issue. High-power onboard chargers or specific radio frequencies caused false short-to-power detection in the second-stage circuit. Fix: Upgraded SRS ECU software to V2.1 via BYD VDS online programming and checked airbag ground harness tightness. No recurrence after upgrade.
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.