AU
B178C-00

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1 — Atto 3

Safety System

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1.5–3.0 Ω).

In the BYD dual-stage airbag system, the Stage 2 gas generator provides higher deployment power during a severe collision.

High resistance indicates excessive impedance or a potential open circuit in the ignition circuit.

Potential causes include an internal open circuit in the airbag squib, poor contact at the wiring harness connector, or a clock spring fault.

This safety system hard fault forces the SRS ECU into fail-safe mode, disabling deployment of this airbag stage and severely reducing front passenger protection during a collision.

Repair immediately.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Aging, moisture ingress, or internal open circuit in the front passenger airbag module (PAB) second-stage igniter increases resistance above 3.0Ω.
  • 2Airbag wiring harness connector (usually located under the centre console, behind the glovebox, or at the airbag module interface) is loose, oxidised, has water ingress, or backed-out pins, causing increased contact resistance.
  • 3Broken wire or poor contact in the clock spring (spiral cable) internal second-stage circuit, common on vehicles after turning the steering wheel to its end stop. (Note: On some models, the front passenger airbag also passes through the clock spring or a similar transfer connector.)
  • 4Improper repair after a vehicle accident, such as failing to tighten the connector to the specified torque during previous airbag removal and installation, or using non-genuine parts causing an impedance mismatch.
  • 5Internal airbag ECU A/D sampling circuit fault or software calibration error falsely reporting abnormal resistance (less common, but requires inspection)
What To Do
  • 1
    Safety preparation: Switch the vehicle OFF, disconnect the 12V battery negative terminal and wait at least 3 minutes to fully discharge the SRS backup power supply. Wear an anti-static wrist strap and avoid using radio equipment near the airbag assembly.
  • 2
    Fault confirmation and freeze frame analysis: Use the BYD VDS2000/ED400 diagnostic tool to read DTCs, confirm B178C-00 is a current fault (Active), record the resistance reading and ambient temperature from the freeze frame data, and check for associated fault codes (such as B178B, B178D, etc.).
  • 3
    Wiring harness connector inspection: Verify the front passenger airbag module yellow wiring harness connector (usually located on the right side of the centre console or behind the glovebox) locks securely. Verify the CPA (Connector Position Assurance) clip seats correctly. Inspect the terminals for green oxidation, burn marks, or backed-out pins. Apply electronic contact cleaner if necessary.
  • 4
    Airbag module resistance measurement: Disconnect the battery first, then remove the front passenger airbag module following the standard removal procedure. Use a digital multimeter to measure the resistance between the two terminals of the airbag second-stage igniter. Normal resistance is 1.5-2.5 Ω. If the resistance is >3.0 Ω or displays OL (open circuit), replace the front passenger airbag assembly with a genuine part.
  • 5
    Harness continuity test: Inspect the harness from the SRS ECU to the airbag (including the floor and instrument panel harnesses). Measure the end-to-end resistance; it must be <1.0 Ω. Check for shorts to ground or power. Carefully inspect the harness sections passing through the firewall and hinge areas for wear or crush marks.
  • 6
    Replacement verification and system reset: If the wiring harness and airbag test normal, replace the SRS ECU with a known good unit for verification. After repair, reassemble the components and connect the battery. Use the diagnostic tool to clear the fault code and perform the 'SRS system self-test'. Verify the airbag warning light turns off after 6 seconds and confirm the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

2018 Qin EV450: Front passenger airbag connector oxidized, causing high resistance

✓
After driving through water, the instrument panel airbag warning light remained on constantly. DTC B178C-00 stored. The technician removed the passenger glove box and found visible water stains and oxidation inside the yellow airbag harness connector. They cleaned the terminals with precision electronic cleaner, applied conductive protectant, and reconnected the connector. Resistance returned to 2.1Ω. After clearing the fault codes, the system returned to normal. Inspect the A/C drain hose and windshield sealing to prevent further water ingress.
BYD DTC AI Analysis

2017 Qin 100 — Clock spring internal open circuit caused second-stage airbag fault

✓
Customer reported the airbag warning light occasionally comes on when turning. Diagnosis found intermittent code B178C-00. Inspected the passenger airbag wiring harness and found it normal, but noted this model's passenger airbag circuit passes through a connector under the center console (functionally equivalent to a clock spring). Disassembly showed internal contact spring fatigue causing unstable contact resistance (normal when cold, increasing when hot or under vibration). Replaced the instrument panel wiring harness assembly and cleared the fault.
BYD DTC AI Analysis

BYD Qin EV300 2017 — Airbag module internal igniter degradation

✓
Purchased used vehicle with airbag warning light on; DTC B178C-00 present. Harness side connector resistance measured normal (<1Ω), but airbag module side second stage igniter resistance was 5.8Ω (severely high). Vehicle had been parked long-term in hot, humid conditions, causing the internal igniter to oxidize from moisture. Replaced genuine passenger side airbag module (part number matched to vehicle VIN), performed VCU and SRS system matching and programming. Fault cleared.
BYD DTC AI Analysis

2017 Qin 80: Connector not locked after accident repair

✓
After front-end collision repairs, the airbag warning light was on at delivery. Read DTC B178C-00. Inspection found the passenger airbag had been removed during accident repairs. The wiring harness connector wasn't fully seated on reinstallation—the CPA retainer didn't click, causing poor pin contact. Reconnected the connector firmly, confirmed it locked, cleared the DTC and verified it didn't return. Always perform a comprehensive SRS self-check after accident repairs.
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.