AU
B178A-00

DTC B178A-00 indicates the SRS (airbag) electronic control unit detects an abnormally low resistance (close to 0 ohms) in the front passenger Stage 2 frontal airbag ignition circuit, indicating a short circuit (short to ground or short between wires) — Atto 3

Safety System

DTC B178A-00 indicates the SRS (airbag) electronic control unit detects an abnormally low resistance (close to 0 ohms) in the front passenger Stage 2 frontal airbag ignition circuit, indicating a short circuit (short to ground or short between wires).

In a dual-stage ignition system, the Stage 2 airbag provides additional protection during severe collisions, typically deploying simultaneously with Stage 1 or triggering with a delay.

A resistance of 0 ohms causes the SRS ECU to determine the ignition circuit is unreliable, illuminate the airbag warning light, and disable the front passenger airbag Stage 2 deployment function.

In extreme cases, the airbag may deploy inadvertently without a collision or fail completely.

This is a hard fault.

The ECU stores it continuously until repaired.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Front passenger airbag module second-stage igniter internal short circuit (damaged internal coil insulation causing a short circuit between positive and negative terminals)
  • 2Airbag wiring harness short to body ground (harness wear, crushing, or aging damages insulation, causing contact with the metal frame)
  • 3Water ingress or corrosion at the front passenger airbag connector (A/C condensate leak or wading causing a short circuit between connector pins)
  • 4SRS control unit internal monitoring circuit fault (damaged ECU internal sampling resistor or comparator circuit causing false report)
  • 5Improper repair procedures (measuring airbag pins directly with a standard multimeter, causing an internal short circuit, or damaging the wiring harness with tools during removal and installation)
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (some models require 3 minutes) to fully discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Fault confirmation: Use a BYD dedicated diagnostic tool (VDS or ED400) to read and record the fault code. Confirm B178A-00 is a current fault (Active). Check the environmental conditions recorded in the freeze frame data.
  • 3
    Visual inspection: Check the front passenger airbag module, the wiring harness under the instrument panel, and the floor harness connector (usually located on the right side of the centre console or behind the glove box) for obvious damage, water stains, or burn marks.
  • 4
    Resistance measurement: Disconnect the airbag connector. Use a dedicated airbag tester (or high-impedance digital multimeter) to measure the resistance between the Stage 2 inflator pins. The normal value is 1.5-3.0 Ω. If the resistance is 0-0.5 Ω, replace the airbag module.
  • 5
    Harness continuity test: Measure the resistance to ground of the second-stage circuit at the harness-side connector (should be >10 kΩ or infinite). Measure wire-to-wire insulation (resistance between positive and negative wires should be infinite). Focus inspection on the harness fixing points routed through the center console.
  • 6
    Simulation verification: Replace the actual airbag with a 2Ω airbag simulator, clear the fault code, turn on the power, and perform an SRS self-check. If the fault code disappears, the wiring harness and ECU are normal; the fault is in the airbag module.
  • 7
    Component replacement and verification: Based on the diagnostic results, replace the front passenger airbag module (use new clips and guide sleeves), repair the wiring harness, or replace the SRS ECU. After reassembly, perform 'SRS System Configuration' and 'Crash Sensor Calibration' (if applicable). Finally, perform a vehicle crash simulation test (use the diagnostic tool to trigger the test function).
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV300 front passenger airbag second-stage circuit short after water fording

✓
A 2017 Qin EV300 showed a steady SRS warning light after wading through heavy flood water. Retrieved DTC B178A-00 (current fault). Stripped and inspected the front passenger floor wiring harness; the waterproof grommet sealed poorly. Water ingress oxidised and shorted the airbag connector pins. Repair: Thoroughly dried the harness, cleaned the connector with electronic contact cleaner, applied conductive anti-corrosion grease, and replaced the damaged airbag wiring harness branch. After reassembly, cleared the DTCs and the SRS system self-test passed. Fault resolved.
BYD DTC AI Analysis

Qin 100 dashboard wiring harness pinched and shorted after accident repair

✓
After the accident-damaged vehicle had its front bumper, dashboard and passenger airbag replaced, it immediately set DTC B178A-00 on startup. Inspection revealed the technician failed to route the passenger side second-stage airbag harness along its original path when installing the dashboard. The harness became pinched against the metal edge of the dashboard frame, chafing through the insulation and causing the positive and negative conductors to short together. Repair: Removed the damaged section of wiring, soldered the connections and insulated with heat-shrink tubing. Re-routed and secured the harness according to factory specifications, keeping it away from sharp edges and high-temperature components. Fault resolved.
BYD DTC AI Analysis

Manufacturing defect in the airbag module internal igniter caused zero resistance.

✓
A 2018 BYD Qin EV450's SRS warning light illuminated suddenly at 20,000 km without collision. The diagnostic scanner showed static fault code B178A-00. Disconnecting the airbag connector revealed 0.3Ω resistance at the module-side second-stage pins (normal ~2.0Ω). This indicated an internal short caused by a powder pressing defect in the second-stage igniter. Replaced the passenger airbag assembly (including second-stage inflator) with a new unit, reinstalled using special tools with torque set to 9-10 N·m, then programmed and matched the module. Fault cleared.
BYD DTC AI Analysis

False positive fault in SRS ECU internal sampling circuit

✓
Intermittent DTC B178A-00; normal when cold, present when hot. Measured airbag module and wiring harness resistance, both normal at 1.8Ω; harness insulation good. Suspected poor thermal stability of internal A/D converter circuit or sampling resistor in SRS ECU. Replaced airbag control unit (located beneath centre console), performed ECU coding (entered vehicle VIN and airbag configuration code) and system initialisation learning. Road test confirmed fault did not recur.
BYD DTC AI Analysis

Bent connector pin caused positive and negative terminals to contact

✓
The fault appeared after routine maintenance at the dealership. Root cause: the technician bumped the passenger airbag connector while removing the glovebox to check the cabin filter, bending the second-stage circuit pins until they contacted and shorted. Fix: Disconnected the battery, carefully straightened the pins, confirmed no fractures, reseated the connector, applied contact protectant, and cleared the codes. Protect SRS connectors during service.
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.