AU
B178D1B

This fault code indicates an open circuit or abnormal resistance in the Stage 2 squib circuit of the front passenger dual-stage frontal airbag (sub-code 1B typically indicates an open circuit or excessive resistance) — Atto 3

Safety System

This fault code indicates an open circuit or abnormal resistance in the Stage 2 squib circuit of the front passenger dual-stage frontal airbag (sub-code 1B typically indicates an open circuit or excessive resistance).

In the BYD SRS system, dual-stage airbags feature two independent squibs (Stage 1 for moderate collisions, Stage 2 for severe collisions; the stages can deploy simultaneously or sequentially).

B178D1B indicates the ECU detects the Stage 2 deployment circuit resistance falls outside the calibrated range (typically 2.0–3.0 Ω), showing infinite resistance (open circuit) or intermittent high resistance.

During a severe frontal collision, this fault prevents the passenger airbag from deploying the second inflation stage as designed, reducing occupant protection.

The SRS system enters fail-safe mode and continuously illuminates the instrument panel airbag warning light.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Front passenger airbag module second-stage plug (yellow connector) not fully seated or locking tab loose. Commonly caused by failing to reconnect the plug after dashboard removal and installation, or missing it during new vehicle PDI.
  • 2Airbag wiring harness open circuit or excessive contact resistance. This frequently occurs when long-term vibration fractures the harness behind the instrument panel at a frame bend, or due to oxidized connector terminals or terminal back-out.
  • 3Airbag module second-stage squib failure or open circuit. This internal module fault cannot be repaired. Replace the complete assembly.
  • 4Water ingress, corrosion, or deformed pins at the SRS ECU to airbag wiring harness connector (e.g., dashboard harness to floor harness inline connector), causing signal interruption.
  • 5Connecting only the first-stage squib and failing to connect the second-stage connector during accident repairs, or using a non-genuine airbag, causing a resistance mismatch.
What To Do
  • 1
    Perform the high-voltage and airbag power-down procedures: disconnect the battery negative terminal and wait at least 3 minutes (to fully discharge the SRS capacitor). Do not operate any electrical switches during this time.
  • 2
    Visual inspection: Remove the glovebox or lower dashboard trim panel. Verify the yellow passenger airbag module connector (usually located behind the right side of the dashboard) is fully seated. Confirm the dual locking mechanism (CPA lock) is fully engaged. Inspect the connector for water ingress or burn marks.
  • 3
    Resistance measurement: Disconnect the airbag module connector. Use a digital multimeter to measure the Stage 2 circuit resistance on the wiring harness side (refer to the vehicle wiring diagram for the correct pins, typically two specific pins on the airbag connector). Normal value: 2.0-3.0 Ω. A reading of OL (open circuit) or >10 Ω indicates an open circuit in the wiring harness or poor connector contact.
  • 4
    Harness continuity check: Measure harness continuity section by section from the SRS ECU (usually located under the center console or at the bulkhead) to the front passenger airbag module. Inspect for wear behind the instrument panel frame and at the harness retaining clips. Repair any open circuits and apply insulation and anti-abrasion protection.
  • 5
    Airbag module inspection: If the wiring harness tests normal (resistance <1Ω and good insulation to ground), the fault is an internal failure of the second-stage squib in the airbag module. Replace the front passenger airbag assembly (Note: Do not measure resistance on the airbag module side to prevent accidental deployment).
  • 6
    System reset verification: Restore all connections and turn on the power. Use the VDS2000 or a Launch/Autel diagnostic tool to clear the fault code. Perform an SRS system self-test. Confirm B178D1B does not return and the instrument panel airbag warning light turns off. Read the data stream to confirm the second-stage circuit resistance is within the normal range.
Real-World Cases
BYD DTC AI Analysis

Front passenger airbag stage 2 circuit found disconnected during new vehicle PDI.

✓
During pre-delivery PDI on a 2020 BYD E3, the dashboard airbag warning light stayed on. DTC B178D1B logged. Removing the right-hand dashboard trim panel revealed the passenger airbag second-stage igniter connector (yellow 2-pin) seated with the CPA lock unengaged, causing intermittent terminal resistance. Re-seated the connector, engaged the lock, and cleared the codes. System returned to normal. Factory assembly oversight.
BYD DTC AI Analysis

Second-stage connector left disconnected after accident repair

✓
A 2019 Qin EV was in a frontal collision. A shop replaced the front passenger airbag assembly. After the repair, the airbag warning light stayed on with DTC B178D1B. The technician found that the repairer had only connected the first-stage (primary) airbag connector, leaving the second-stage connector disconnected behind the dashboard. Dual-stage airbags have two separate connectors (or two sets of terminals within a composite connector). Reconnecting the second-stage connector and securing the wiring harness cleared the fault.
BYD DTC AI Analysis

Dashboard removal and refitting pinched the wiring harness, causing an open circuit.

✓
A BYD E2 developed an intermittent B178D1B fault after technicians removed and refitted the dashboard to repair a leaking AC evaporator. They found a screw had pinched the passenger airbag harness during dashboard frame installation, partially fracturing the copper wires and leaving only a few strands intact. Vehicle vibration broke the connection, causing an open circuit. They freed the harness, soldered and insulated the broken wires with heat-shrink tubing, adjusted the routing to prevent interference, and completely resolved the fault.
BYD DTC AI Analysis

Second-stage squib failure in airbag module

✓
A Qin EV with 80,000 km and no accident history suddenly showed the airbag warning light. DTC B178D1B appeared and would not clear. Continuity from the SRS ECU to the airbag connector measured 0.3Ω (normal), with good ground insulation. The airbag module's internal second-stage squib had aged and failed open. Replaced the passenger frontal airbag assembly (including the second-stage airbag module). Programmed and matched the new module; the DTC cleared and the system self-test passed.
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.