AU
B178C

This DTC indicates the SRS (Supplemental Restraint System) ECU detected the circuit resistance of the front passenger-side second-stage frontal airbag (a dual-stage inflator using staged ignition technology) exceeds the calibrated upper limit (normal value approximately 2 — Atto 3

Safety System

This DTC indicates the SRS (Supplemental Restraint System) ECU detected the circuit resistance of the front passenger-side second-stage frontal airbag (a dual-stage inflator using staged ignition technology) exceeds the calibrated upper limit (normal value approximately 2.0-3.0Ω, fault threshold typically >3.5Ω or open circuit).

The second-stage airbag deploys in stages based on the collision severity sensor signal (delayed ignition) to reduce occupant impact during a low-speed collision.

Excessive resistance indicates a high-resistance connection or potential open circuit.

Oxidized or corroded connectors, loose wiring harness connections, fatigue fracture of the spiral resistance wire inside the airbag inflator, or an abnormal ECU sampling circuit can cause this condition.

This fault causes the airbag to deploy only in single-stage mode during a collision (delayed ignition failure) or fail completely, severely degrading crash protection performance.

This is a safety-critical fault.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Front passenger airbag module wiring harness connector loose, oxidized, or making poor contact (common in water-damaged vehicles or after prolonged use in high-humidity environments)
  • 2The airbag wiring harness pinches and chafes where it passes through the center console frame or near the steering column, breaking the copper strands without completely damaging the insulation (hidden high resistance).
  • 3The resistance wire inside the second-stage inflator igniter broke due to aging, or a solder joint has a poor connection (internal airbag assembly fault; non-repairable, replace the complete assembly).
  • 4Vehicle modifications (such as installing a central display or dash cam) damaged the SRS wiring harness, or the connector is not fully seated.
  • 5Airbag ECU internal A/D conversion circuit or sampling resistor fault (confirm by bridging a standard resistor to isolate the fault)
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes to fully discharge the SRS capacitor and enter safe repair mode.
  • 2
    Initial inspection: Remove the front passenger glove box or lower airbag trim panel. Visually inspect the yellow SRS connector for backed-out pins or oxidation (green corrosion). Measure the resistance between the connector terminals (standard: 2.0-3.0 Ω).
  • 3
    Wiring inspection: If resistance exceeds specifications, disconnect the airbag connector and bridge the airbag-side connector with a jumper wire. Measure circuit continuity (<1Ω) and insulation to ground (>1MΩ) at the ECU end. Thoroughly inspect the wiring harness sleeve at the edge of the center console frame.
  • 4
    Component replacement: If the wiring is normal but the airbag-side resistance is >3.5Ω, the second-stage inflator has an internal fault. Replace the front passenger airbag assembly. (Note: Never measure airbag resistance directly using a multimeter in resistance mode. Use a dedicated airbag simulator.)
  • 5
    System verification: Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform the 'SRS system self-test' and verify it passes. Finally, perform a static crash simulation test (use a 2.7Ω substitute resistor to verify ECU recognition).
Real-World Cases
BYD DTC AI Analysis

Intermittent B178C fault on Song MAX after water wading

✓
A 2019 Song MAX showed an intermittent airbag warning light after driving through heavy rain and standing water. The scanner read B178C (current fault). Technicians found the passenger airbag connector under the center console had retained floodwater, causing slight pin oxidation (grey-green rust spots). Continuity remained, but contact resistance measured 4.2Ω. Cleaning the connector and applying specialized conductive paste fixed the fault. Seal SRS connectors on any vehicle that has been through flood water.
BYD DTC AI Analysis

Yuan EV second-stage airbag resistance high after accident repair

✓
2019 BYD Yuan EV. DTC B178C set three months after front collision repairs. The shop replaced the passenger airbag using an aftermarket wiring harness. Inspection found a section of secondary airbag circuit harness pinched by the sharp edge of the center console frame, causing 50% of internal copper wires to fracture and high resistance (3.8Ω). Replaced with genuine SRS harness and rerouted to avoid interference. Fault resolved. Note: Pay attention to harness routing and protection during accident repairs.
BYD DTC AI Analysis

Tang DM aftermarket large screen installation caused loose connector

✓
2021 BYD Tang DM. One week after the owner fitted an aftermarket 12.3-inch centre touchscreen, a B178C fault appeared. Inspection revealed the installer had tugged on the SRS wiring harness behind the glove box during the retrofit, causing the passenger-side second-stage airbag connector (normally concealed behind the glove box) to work loose by approximately 2 mm. While not fully disconnected, this increased contact resistance. The technician reseated the connector and secured it with an additional clip. The fault code cleared and has not returned. When removing or refitting the centre console, check the SRS connector condition.
BYD DTC AI Analysis

Airbag gas generator internal resistance wire fractured

✓
A Qin Pro EV with 80,000 km and no accident history logged DTC B178C. Wiring continuity checked normal. Direct measurement at the airbag assembly connector showed infinite resistance (open circuit). Diagnosis: fatigue fracture of the igniter resistance wire inside the second-stage gas generator—an internal airbag assembly fault. Replaced the passenger side airbag assembly with a new unit (part number must match vehicle configuration; distinguish between versions with and without side airbags). Programming and matching cleared the fault. Typical component aging case.
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.