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 8

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 8
  • 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 typically occurs when the harness behind the instrument panel fractures at a frame bend after prolonged vibration, or when connector terminals oxidize or back out.
  • 3Failure or open circuit in the airbag module second-stage igniter (Squib). This internal module fault is unrepairable. Replace the complete assembly.
  • 4Water ingress, corrosion, or deformed pins in the wiring harness connector between the SRS ECU and the airbag (e.g., the mating connector between the instrument panel harness and floor harness), causing signal interruption.
  • 5During accident repairs, connecting only the first-stage squib and leaving the second-stage connector unplugged, or installing a non-genuine airbag, causes a resistance mismatch.
What To Do
  • 1
    Perform the high-voltage power-down and airbag power-down procedure: 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. Check if the yellow passenger airbag module connector (usually located behind the right side of the dashboard) is fully seated. Verify the dual locking mechanism (CPA lock) is fully engaged. Check the connector for water ingress or burn marks.
  • 3
    Resistance measurement: Disconnect the airbag module connector and use a digital multimeter to measure the second-stage circuit resistance on the wiring harness side (check pins against the vehicle wiring diagram; usually two specific pins on the airbag connector). The normal value is 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 centre console or 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 treatment.
  • 5
    Airbag module inspection: If wiring harness measurements are normal (resistance <1Ω and good insulation to ground), diagnose an internal fault in the airbag module second-stage igniter. Replace the front passenger frontal 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 Launch/Autel diagnostic tool to clear the fault code. Perform an SRS system self-check. Verify 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 8 Fault Codes
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.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.