AU
B1787

DTC B1787 indicates an open circuit fault in the second-stage deployment circuit of the driver-side dual-stage front airbag — Atto 8

Safety System

DTC B1787 indicates an open circuit fault in the second-stage deployment circuit of the driver-side dual-stage front airbag.

A dual-stage airbag contains two independent inflation stages: the first stage (low-output deployment) and the second stage (high-output deployment).

The second stage typically deploys with a delay or provides greater inflation volume during severe collisions.

This fault code indicates the SRS control unit (ACU) detects second-stage deployment circuit resistance exceeding the threshold (typically >10Ω or open circuit), preventing proper second-stage airbag deployment during a collision.

This fault does not disable the first-stage airbag, but it eliminates the staged protection function, increasing the risk of occupant injury during a severe collision.

An electrical continuity interruption in the circuit causes this fault.

The interruption may occur in the clock spring (spiral cable), airbag assembly, wiring harness connector, or inside the SRS ECU.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Clock spring (spiral cable) internal break: Frequent steering wheel rotation causes fatigue fracture of the internal flat wiring harness, resulting in an open circuit, particularly in the Stage 2 circuit wire.
  • 2Airbag assembly connector loose or oxidized: Yellow connector behind the steering wheel not fully latched, or terminal oxidation or water ingress corrosion causes excessive contact resistance.
  • 3Airbag inflator second-stage circuit internal open circuit: Igniter (squib) inside the airbag unit burned out or internal connection disconnected.
  • 4Steering column wiring harness wear: Prolonged chafing of the wiring harness inside the steering column damages insulation or breaks wires.
  • 5SRS control unit internal drive circuit fault: Damaged ACU internal ignition driver chip or dry solder joint on related pins.
What To Do
  • 1
    Read the DTC freeze frame data using the BYD dedicated diagnostic tool (VDS2000/VDS2100). Record the vehicle speed, crash signal status, and voltage values at the time of the fault. Clear the DTC and road test the vehicle to verify if the fault recurs.
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes. Remove the covers on both sides of the steering wheel. Check the connection of the driver airbag yellow connector. Verify the locking tab is fully engaged and free of oxidation or terminal back-out. If necessary, clean the connector and apply conductive grease.
  • 3
    Remove the steering wheel (using the special tool) and check the continuity of the clock spring (spiral cable): Use a multimeter to measure the resistance of the clock spring corresponding to the second-stage circuit. Normal resistance is <1Ω. Check continuity stability while rotating the clock spring.
  • 4
    Measure the airbag assembly second-stage circuit resistance: Disconnect the airbag connector and measure the second-stage igniter resistance at the airbag side. The normal value is 2.0-3.0 Ω (BYD standard). If the reading is OL or >10 Ω, replace the airbag assembly.
  • 5
    Check the SRS wiring harness: Perform a continuity test on the harness from the ACU to the steering wheel. Specifically inspect the wires inside the harness sleeve below the steering column for wear or breakage. Measure the end-to-end harness resistance; it must be <1Ω.
  • 6
    If all above checks are normal, inspect the SRS control unit (ACU) connector for oxidized or backed-out pins. If necessary, use an oscilloscope to test the ACU ignition diagnostic signal output waveform. Replace the control unit after confirming an internal ACU fault.
  • 7
    After repair, reconnect all components and the battery. Use the diagnostic tool to perform 'SRS System Configuration' and 'Sensor Calibration' (if applicable). Clear the fault codes and perform a system self-check to confirm B1787 does not return.
Real-World Cases
BYD DTC AI Analysis

BYD Tang DM clock spring failure caused DTC B1787

✓
2021 BYD Tang DM, 60,000 km. SRS warning light illuminated. Retrieved DTCs B1787 (Driver second-stage frontal airbag disconnected) and B1786 (Excessive resistance). Checked airbag connector – normal. Measured clock spring second-stage circuit resistance – infinite (open circuit). Removed the clock spring and found the internal flat ribbon cable fractured at the steering wheel center position. Replaced with OEM clock spring (Part No.: SC-3658300). Resistance returned to normal 2.3Ω. Cleared DTCs and road tested – fault resolved. Cause: frequent steering while stationary caused internal wiring fatigue.
BYD DTC AI Analysis

BYD Yuan EV Steering Wheel Wiring Harness Wear Case

✓
2019 BYD Yuan EV developed a B1787 fault after accident repair. Inspection found steering wheel impact during the accident damaged the wiring harness sleeve under the steering column; the internal second-stage airbag harness had 90% of copper strands broken with only a few still connected. Re-stripped and soldered the wires, reinforced insulation with heat-shrink tubing, and rerouted the harness to avoid chafing against the steering column metal edges. Measured circuit resistance at 1.8Ω after repair. Fault code cleared and has not returned. Check steering column harness integrity when repairing accident-damaged vehicles.
BYD DTC AI Analysis

BYD Song MAX airbag connector oxidized, poor contact

✓
2019 BYD Song MAX used in humid southern regions. Airbag warning light illuminated intermittently; DTC B1787 intermittent. Removed and inspected the airbag connector beneath the steering wheel; found green corrosion on the internal terminals of the yellow connector and a perished waterproof sealing ring. Cleaned the terminals with electronic contact cleaner, applied specialized conductive anti-corrosion grease, replaced the waterproof sealing ring, and reconnected. Secured the connector with cable ties to prevent loosening. Monitored for one month with no fault recurrence. Vehicles in humid regions require regular inspection of airbag connector seals.
BYD DTC AI Analysis

Internal open circuit in BYD Qin Pro airbag assembly

✓
A 2020 Qin Pro showed an airbag warning on the dash after a minor rear-end collision. The scan tool read DTC B1787, which would not clear. The clock spring and wiring harness tested normal, but the second-stage circuit resistance at the airbag assembly connector measured open (infinite ohms). Removed the airbag assembly (disconnect battery negative and wait 3 minutes first), disassembled it, and found the second-stage igniter pin on the gas generator had detached from its solder joint due to impact shock. Replaced the driver airbag assembly (including the gas generator) and the fault cleared. This is a rare product quality case; check airbags from the same batch.
Other Atto 8 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.