AU
B178D

DTC B178D indicates the front passenger second-stage frontal airbag (Passenger Frontal Airbag Stage 2) ignition circuit is disconnected or has abnormal resistance — Atto 3

Safety System

DTC B178D indicates the front passenger second-stage frontal airbag (Passenger Frontal Airbag Stage 2) ignition circuit is disconnected or has abnormal resistance.

In BYD e-Platform 3.0 models featuring a distributed electronic architecture (e.g., Dolphin, Seal, Yuan PLUS), this DTC may also indicate a communication interruption between the Right Body Domain Controller (R-DCU) and the SRS module, or a sub-node fault.

The airbag utilizes a dual-stage ignition design to deploy in stages based on collision severity (stage 1 low-pressure deployment; stage 2 high-pressure supplementary deployment).

This fault prevents the front passenger airbag from executing the second-stage deployment during a collision or causes complete airbag failure, severely compromising occupant protection.

Inspect the airbag module wiring harness, connectors, clock spring (if applicable), and domain controller communication status.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1The front passenger airbag stage 2 ignition circuit is open or its resistance exceeds the threshold (>4.5Ω), preventing the SRS control unit from detecting a valid load.
  • 2Loose airbag wiring harness connector, oxidation from water ingress, or terminal pin back-out (focus inspection on the airbag connector inside the dashboard and the domain controller interface).
  • 3Internal open circuit or poor contact in the clock spring (for models with a conventional clock spring design)
  • 4Internal, software, or power supply fault in the Right Body Domain Controller (R-DCU) causing loss of communication with the SRS subsystem (for vehicles with domain control architecture).
  • 5Internal igniter fault or internal open circuit in the SRS control unit or airbag module.
What To Do
  • 1
    Use the VDS diagnostic tool to read the DTC freeze frame data, confirm the fault status as current (Active) or historical (History), and check for other related fault codes (such as B178C or U0187).
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes to ensure complete system discharge. Check the front passenger airbag module connector (located inside the right side of the instrument panel or at the right domain controller interface) for looseness or oxidation. Measure the second-stage ignition circuit resistance (standard value: 2.0-3.0 Ω).
  • 3
    Check the SRS control unit (or the SRS module integrated in the right domain controller) for power supply (constant B+, IG power), ground, and CAN line voltages (CAN-H: 2.5–3.5 V, CAN-L: 1.5–2.5 V, static differential voltage approximately 0 V).
  • 4
    If the vehicle uses a domain control architecture (such as e-Platform 3.0 models), check the communication status between the right body domain controller and the SRS module. Measure the CAN line terminal resistance at diagnostic connector pins 6 and 14 (approximately 60 Ω). Check if the right domain controller software requires an upgrade.
  • 5
    Check the airbag wiring harness for continuity, focusing on wear, pinching, and poor connector contact in the dashboard wiring harness. Repair the wiring harness or replace the airbag module if necessary.
  • 6
    After replacing the airbag module or domain controller, write the SRS system configuration, calibrate the safety system, and perform an airbag deployment test (simulate using dedicated equipment; do not perform a live test).
Real-World Cases
BYD DTC AI Analysis

BYD Song PLUS DM-i Blind Spot Monitoring Restricted (Right Domain Controller Fault)

✓
Symptom description: Dashboard displayed "Blind Spot Function Limited" warning; blind spot monitoring switch not shown on centre touchscreen; VDS failed to detect right blind spot monitoring module. Diagnostic process: 1. Scanned with VDS; right blind spot monitoring module had no communication. 2. Inspected right body controller (right domain) and confirmed the module communicates via this unit. 3. Checked power supply, ground and CAN wiring to right domain controller; found internal fault in the controller was preventing the blind spot module from working. 4. Cross-check: Swapped right body controller and fault disappeared. Resolution: Replaced right body domain controller (right domain), reprogrammed and cleared the fault.
Original source ↗
BYD DTC AI Analysis

BYD Song Plus DM-i Multiple Switch Group Failure (Right Domain Communication Fault)

✓
Symptoms: Light switch would not turn on lights, wiper switch would not control wipers, and all steering wheel buttons did not respond. Instrument cluster displayed "Lane Departure Function Limited" and "Lane Keeping Function Limited". Diagnosis: 1. VDS scan found communication errors between left and right domain controllers. 2. Multiple switch signals route through the body domain controller; identified as inter-domain controller communication failure. 3. Checked power supply and network wiring to domain controllers; measured abnormal voltage on CAN lines. 4. Confirmed right body domain controller had corrupted internal software, blocking switch signal transmission. Resolution: Updated software on right body domain controller. Fault cleared after update. Some vehicles require replacement of the right domain controller.
Original source ↗
BYD DTC AI Analysis

BYD e2 AC not cooling (Integrated Body Controller/Right Domain fault)

✓
Symptom: AC stops cooling after running for a while. No cold air even with temperature set to minimum. Diagnosis: 1. VDS scan showed Integrated Body Controller logged fault code "AC line in high pressure or low pressure state". 2. AC pressure sensor and wiring checked normal. 3. Found Integrated Body Controller (AC controller integrated in right domain) outputting abnormal control signal to electronic expansion valve. 4. Data stream showed abnormal duty cycle from right domain controller driving expansion valve. Repair: Replaced Integrated Body Controller (right domain). Recalibrated AC system. Fault resolved.
Original source ↗
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.