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 U

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 — Seal U
  • 1Front passenger airbag module second-stage plug (yellow connector) not fully seated or locking tab loose. Common causes include failing to reconnect the plug after dashboard removal/installation, or an omission during new vehicle PDI.
  • 2Airbag wiring harness open circuit or excessive contact resistance. This frequently occurs when long-term vibration breaks the harness behind the instrument panel at a frame bend, or when connector terminals oxidize or back out.
  • 3The airbag module internal second-stage squib has failed or is 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 (such as the dashboard harness to floor harness connector), 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 and airbag power-down procedures: disconnect the battery negative terminal and wait at least 3 minutes (to ensure the SRS capacitor fully discharges). 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. Inspect 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, typically 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 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 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 and 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 switch on the power. Use a VDS2000 or Launch/Autel diagnostic tool to clear the fault code. Perform an SRS system self-check. Confirm DTC 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 Seal U 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.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
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.