AU
B178D-00

DTC B178D-00 indicates the SRS (Supplemental Restraint System) control module detects an open circuit or disconnection in the front passenger-side dual-stage frontal airbag second-stage inflator circuit — Seal U

Safety System

DTC B178D-00 indicates the SRS (Supplemental Restraint System) control module detects an open circuit or disconnection in the front passenger-side dual-stage frontal airbag second-stage inflator circuit.

The dual-stage airbag system contains two independent inflator trigger circuits (stage 1 and stage 2) that deploy in stages based on collision severity.

This fault code specifically indicates the second-stage inflator circuit resistance falls outside the normal range (typically > 3Ω or < 1Ω), causing the system to register a 'not connected' state.

This fault may prevent the front passenger airbag second-stage inflator from deploying correctly during a collision, reducing protection effectiveness, and illuminates the airbag warning light continuously.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger airbag module connector (located behind the instrument panel) loose, pins backed out, or corroded. This often results from failing to seat the connector correctly after previous instrument panel repairs.
  • 2Open circuit or abnormal resistance in the second-stage inflator squib inside the airbag module, usually resulting from airbag aging, moisture ingress, or failure to replace the module after a previous collision.
  • 3Poor contact at the intermediate connector between the floor wiring harness and the instrument panel airbag wiring harness, especially pin oxidation after vehicle wading.
  • 4Internal drive circuit fault in the SRS control module prevents correct identification of the Stage 2 inflator load.
  • 5Physical damage to the wiring harness, such as accidentally cutting or crushing the airbag harness when retrofitting a dashcam, audio system, or air conditioning system.
What To Do
  • 1
    Use the VDS2000/BYD dedicated diagnostic tool to read fault codes, confirm B178D-00 is a current code, record freeze frame data, and check for accompanying communication fault codes.
  • 2
    Disconnect the 12V battery negative terminal and wait at least 90 seconds. Remove the front passenger lower instrument panel trim. Verify the white 2-pin airbag module connector (usually marked PAB Stage 2) is fully locked. Inspect the terminals for oxidation, backing out, or enlarged sockets.
  • 3
    Use a multimeter to measure the resistance between the airbag connector terminals. The normal value is 2.0-3.0Ω (refer to the vehicle repair manual for specific values). If the resistance is infinite, the fault is inside the airbag module. If the resistance is normal, the fault is in the wiring harness or ECU.
  • 4
    Disconnect the SRS ECU connector. Measure the harness continuity between the ECU and the airbag connector (should be < 1Ω) and the insulation to ground and power supply (should be > 10MΩ). Check the floor harness intermediate connector (usually under the carpet beneath the centre console).
  • 5
    If the wiring harness has an open circuit, repair or replace the harness. If the airbag module is faulty, replace the front passenger airbag assembly (Note: Use only a brand-new part; do not use dismantled parts). During installation, listen for a 'click' to confirm the connector locks.
  • 6
    Reconnect the battery and clear the fault code. Use the diagnostic tool to perform 'Airbag System Self-diagnosis' or 'Configuration Verification'. Confirm B178D-00 becomes a history code and clear it. Check if the airbag warning light turns off.
  • 7
    Perform a simulated crash test (use the diagnostic tool actuator test function, or lightly tap the vehicle to trigger the vibration sensor, but do not trigger an actual collision) and confirm no fault codes return.
Real-World Cases
BYD DTC AI Analysis

Qin EV450 dashboard connector unsecured after removal and refitting

✓
A 2018 Qin EV450 developed a constant airbag warning light after the AC evaporator was replaced. Retrieved DTC B178D-00. Found that the technician had disconnected the passenger airbag connector during the earlier dashboard removal to gain access. When reinstalling the dashboard, the connector seated without the lock clicking; road vibrations eventually shook it loose. Reconnected the connector and confirmed the lock engaged, cleared the fault codes, and restored normal operation.
BYD DTC AI Analysis

Qin 100 wiring harness corrosion after water wading

✓
2017 BYD Qin 100. DTC B178D-00 set after driving through standing water in the rain. Removed the mating connector between the floor harness and instrument panel harness (located under the passenger floor carpet) and found water inside. The second stage inflator circuit pin (typically yellow) showed severe oxidation with green copper corrosion; contact resistance exceeded 5Ω, triggering an open-circuit fault. Cleaned the pins with precision electrical contact cleaner, applied conductive protective compound, and replaced connector terminals as needed. Fault resolved.
BYD DTC AI Analysis

Front passenger airbag module internal fault

✓
A Qin EV300 still reported DTC B178D-00 after replacing the front passenger airbag assembly following a minor front collision. The replacement unit measured infinite resistance—internal second-stage squib open circuit, confirming a defective part. Installed another new genuine module: resistance measured 2.3Ω and the fault cleared. Use only BYD genuine parts; aftermarket airbags can cause compatibility issues.
BYD DTC AI Analysis

Aftermarket stereo installation damaged wiring harness

✓
While self-installing an aftermarket centre touchscreen navigation system in their 2017 Qin 80, the owner routed wiring inside the dashboard using a metal fish tape. The tape snagged and tore the insulation of the front passenger airbag second-stage harness, partially severing the copper conductors and creating an intermittent open circuit. Fault codes appeared intermittently. Stripping back the wiring with harness repair tools revealed the yellow airbag wire cut 3/4 through, with only a few copper strands still connected. The damaged section was replaced, soldered, and double-insulated with heat-shrink tubing. The fault cleared completely.
BYD DTC AI Analysis

SRS ECU internal driver chip failure

✓
A Qin EV450 with 80,000 km repeatedly logged DTC B178D-00. Replaced the airbag and wiring harness; fault remained. Resistance at the airbag side and harness continuity measured normal. With the airbag disconnected, the ECU output terminals showed no load voltage (detection voltage should normally be present). This indicated an internal secondary drive circuit failure in the SRS control module. Replaced the SRS ECU and recoded it using BYD dedicated equipment to write the vehicle VIN and configuration codes. Fault resolved.
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.