AU
B178F

DTC B178F indicates a short to ground in the stage 2 deployment circuit of the front passenger dual-stage frontal airbag — Atto 3

Safety System

DTC B178F indicates a short to ground in the stage 2 deployment circuit of the front passenger dual-stage frontal airbag.

In a dual-stage airbag system, the stage 1 and stage 2 inflators trigger in stages based on collision severity: stage 1 provides basic protection, and stage 2 supplements inflation during a severe collision.

This fault means the SRS control module detects an abnormally low resistance (typically below 1.0 Ω) between the stage 2 inflator circuit and body ground.

Possible causes include an internal short circuit in the clock spring (spiral cable), damaged airbag wiring harness insulation contacting the metal frame, corroded connector terminals causing a ground fault, or an internal short circuit in the airbag module inflator.

This fault prevents the stage 2 airbag from deploying normally, reduces protection performance during a severe collision, forces the SRS system into fail-safe mode, and illuminates the airbag warning lamp.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Broken internal wires or worn insulation in the clock spring (spiral cable) shorting the Stage 2 ignition circuit to the grounded housing, commonly caused by fatigue damage from frequent steering wheel rotation.
  • 2Vibration and friction inside the dashboard or near the A-pillar damage the insulation on the wiring harness between the front passenger airbag module and the SRS ECU, causing it to contact the vehicle body metal frame and short to ground.
  • 3Water ingress in the airbag connector (usually located under the dashboard or at the airbag module interface) due to wading, vehicle washing, or poor sealing causes electrolytic corrosion between the terminals and a short to ground.
  • 4Internal fault in the SRS control module ignition drive circuit causing a false short to ground, or a short circuit between the airbag igniter internal bridge wire and the housing (very rare but possible).
  • 5Improper repair procedures, such as directly measuring the igniter circuit using a multimeter resistance setting (which can damage internal SRS ECU circuits), or incorrectly connecting the airbag wiring harness when installing aftermarket electronic equipment.
What To Do
  • 1
    Safety preparation: Switch the vehicle OFF, disconnect the 12V low-voltage battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use the BYD dedicated diagnostic tool (ED400 or X431) to read the DTC, confirm B178F is a current fault (not a history fault), record freeze frame data, and check for accompanying fault codes (such as B178E, B178D).
  • 3
    Visual inspection: Remove the front passenger side dashboard trim panel. Check the airbag module connector (usually a yellow plug) for looseness, water ingress, or corrosion. Check the wiring harness near the steering column and at the dashboard frame mounting points for signs of chafing.
  • 4
    Circuit measurement: Disconnect the SRS ECU and airbag module connectors. Use a multimeter to measure the resistance to ground of the second-stage igniter circuit (usually the black/yellow wire). Normal resistance is infinity (OL). Continuity indicates a short to ground.
  • 5
    Sectional troubleshooting: If the circuit has continuity, use a clock spring tester or sectional measurement method to separately measure the insulation of the clock spring, instrument panel wiring harness, and floor wiring harness to locate the exact short circuit position.
  • 6
    Component replacement: If the clock spring is faulty, replace the clock spring (center the steering wheel during installation); if the wiring harness is damaged, repair the harness and reinforce the insulation; if suspecting an internal short circuit in the airbag module, replace the front passenger airbag assembly.
  • 7
    System verification: Reconnect all connectors, reconnect the battery negative terminal, clear the fault code using the diagnostic tool, perform the SRS system self-check, verify B178F does not return, and confirm the airbag warning light turns off after the self-check.
  • 8
    Functional test: Perform a static test of the crash sensors and airbag system (without deploying the airbags). Verify the SRS ECU communicates normally with all airbag modules and generates no new fault codes.
Real-World Cases
BYD DTC AI Analysis

Clock spring internal short caused B178F

✓
A 2020 BYD Tang DM with 80,000 km displayed “Please check SRS system.” Found codes B178F and B178E (Stage 2 short to ground and short to power). Disassembly revealed the clock spring (spiral cable) beneath the steering wheel had worn insulation on pins 4 and 5 (Stage 2 igniter circuit) from constant rotation; the wires contacted the metal housing and shorted to ground. Replaced the clock spring (Part No.: KJ-3402100). Igniter resistance measured 2.3Ω. Cleared the codes and the system returned to normal.
BYD DTC AI Analysis

Dashboard wiring harness chafed and shorted to ground

✓
2019 BYD Yuan EV. Airbag warning light illuminated after wading through water. Scan tool showed active fault code B178F. Inspection found the passenger airbag harness chafed through at the right-side dashboard mounting clip due to vibration; worn insulation exposed bare copper, which contacted the metal dashboard bracket causing a short to ground. Water ingress accelerated the harness deterioration. Repair: Wrapped the damaged section with insulation tape, rerouted the harness to avoid sharp metal edges, added corrugated tubing protection, dried the connectors, and cleared the fault.
BYD DTC AI Analysis

Corroded airbag connector due to water ingress

✓
2021 BYD Qin Pro used in a high-rainfall southern region; the passenger-side A-pillar had previous panel beating repairs. The airbag warning light illuminated intermittently, with DTC B178F stored as an intermittent fault. Removed and inspected the passenger airbag connector (located behind the glovebox) and found green corrosion on the terminals with obvious water stains. Poor sealing during the A-pillar repair allowed rainwater ingress. Cleaned the connector terminals, sprayed electronic cleaner, replaced the waterproof seal, and repaired the A-pillar water barrier. The fault has not reoccurred.
BYD DTC AI Analysis

SRS ECU false positive fault

✓
2019 BYD Song MAX. Replaced the passenger airbag after accident repairs, but the airbag warning light remained on with DTC B178F. New airbag secondary igniter resistance measured 2.1Ω (normal). Wiring harness showed good insulation to ground. Suspected SRS control module damage from the accident caused abnormal detection in the internal igniter drive circuit. Replaced the SRS ECU with an identical unit (reconfigured VIN and sensor parameters), cleared the fault code, and confirmed it did not return. Confirmed internal control module failure causing false alarm.
BYD DTC AI Analysis

Wiring error in aftermarket device caused short circuit

✓
2020 BYD Tang. Owner installed a dash cam and mistakenly spliced the power feed into the passenger airbag harness. This grounded the second-stage igniter circuit through the power wire, triggering DTC B178F. Inspection found the airbag harness cut and incorrectly reconnected. Removed the unauthorized wiring and restored the original circuit using a dedicated harness repair kit (waterproof heat-shrink tubing and soldering). Educated the owner that airbag harnesses are marked yellow and must never be modified or tapped into.
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.