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) — Seal 6 EV

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.

The B178D1B fault code on the Seal 6 EV indicates an open circuit or excessive resistance in the Stage 2 circuit of the front passenger dual-stage airbag. Severity is medium because it may stop the second inflation stage deploying in a severe crash. Common causes include a loose yellow airbag connector or damaged/oxidised wiring behind the dashboard. Visit an authorised dealer for SRS diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal 6 EV
  • 1Front passenger airbag module second-stage plug (yellow connector) not fully seated or locking tab loose. This commonly results from failing to reconnect the plug after dashboard removal and installation, or overlooking it during new vehicle PDI.
  • 2Airbag wiring harness open circuit or excessive contact resistance, commonly occurring when the harness behind the instrument panel fractures at a frame bend after prolonged vibration, or when connector terminals oxidize or back out.
  • 3The airbag module internal second-stage squib failed or has an open circuit. This is an unrepairable internal module fault. Replace the complete assembly.
  • 4Water ingress, corrosion, or deformed pins in the SRS ECU to airbag wiring harness connector (such as the mating connector between the instrument panel harness and floor harness), causing signal interruption.
  • 5Connecting only the first-stage squib and leaving the second-stage connector unplugged during accident repairs, or installing 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 glove box or lower dashboard trim panel. Check if the front passenger airbag module yellow connector (usually located on the right side behind the dashboard) is fully seated. Confirm 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. Use a digital multimeter to measure the second-stage circuit resistance on the wiring harness side (refer to the vehicle wiring diagram for the correct pins, 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 center 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 proper insulation and anti-abrasion protection.
  • 5
    Airbag module inspection: If wiring harness measurements are normal (resistance <1Ω and good insulation to ground), the airbag module internal second-stage igniter is faulty. 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 a Launch/Autel diagnostic tool to clear the fault code. Perform an SRS system self-check. Confirm B178D1B does not return and the instrument cluster 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 Seal 6 EV Fault Codes
B1108For 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.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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.