AU
B1787-00

DTC B1787-00 indicates the SRS (Supplemental Restraint System) control module detects an open circuit fault (abnormal resistance or open circuit) in the second-stage firing circuit of the driver-side dual-stage front airbag — Atto 8

Safety System

DTC B1787-00 indicates the SRS (Supplemental Restraint System) control module detects an open circuit fault (abnormal resistance or open circuit) in the second-stage firing circuit of the driver-side dual-stage front airbag.

Modern BYD models use a dual-stage airbag design; the first and second stages correspond to different deployment strategies based on collision severity.

A "not connected" system report indicates the ECU cannot establish a complete circuit path to the second-stage inflator.

This prevents the airbag from executing its staged deployment strategy during a moderate to severe collision, potentially triggering only the first stage or failing completely.

The SRS also illuminates the instrument cluster airbag warning light and may disable the entire driver-side front airbag function, severely compromising crash protection performance.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Internal open circuit or poor contact in the clock spring (spiral cable), especially a broken internal flat ribbon cable caused by prolonged steering.
  • 2Driver airbag module second-stage igniter internal open circuit or resistance out of specification (typical standard value: 1.5-3.0 Ω)
  • 3Loose, oxidized, or water-damaged yellow airbag wiring harness connector below the steering wheel (usually a 2-pin yellow plug), or backed-out terminals.
  • 4Open circuit, corrosion, or deformed pins at the floor wiring harness to steering column wiring harness connector, common in vehicles with water ingress or damp floors.
  • 5Internal fault in the SRS control unit diagnostic circuit (less common; confirm after ruling out wiring faults)
What To Do
  • 1
    Safe power-down: Turn off the ignition switch, disconnect the 12V low-voltage battery negative terminal, and wait at least 90 seconds (to fully discharge the SRS backup power capacitor). Never use a multimeter resistance setting to measure the airbag module directly.
  • 2
    Initial visual inspection: Verify the yellow airbag connector below the steering wheel is fully locked, inspect the wiring harness for signs of abrasion, crushing, or rodent damage, and check the clock spring housing for damage.
  • 3
    Sectional resistance measurement: Remove the driver airbag (wait at least 90 seconds after disconnecting the battery). Use a digital multimeter to measure the resistance between the second-stage terminals of the airbag unit (specified value: 1.5-3.0 Ω). If normal, measure the continuity of the wiring harness from the vehicle side of the clock spring to the SRS ECU.
  • 4
    Clock spring inspection: Disconnect the connectors at both ends of the clock spring and measure the continuity of the internal second-stage circuit. If resistance is infinite or unstable, replace the clock spring (part numbers usually indicate 'Clock Spring' or 'spiral cable').
  • 5
    Harness repair: If the connector is loose, reconnect it and secure the locking clip. If terminals are oxidized or pins have backed out, use a dedicated terminal removal tool to repair them or replace the wiring harness assembly. Check the mating connector between the floor harness and the steering column harness (usually located behind the trim panel below the steering wheel).
  • 6
    System reset and verification: After repair, reconnect all components and restore the battery connection. Use the diagnostic tool to clear the fault code. Perform the 'SRS system self-check' or 'configuration and calibration' procedure. Confirm B1787-00 does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV300 Clock Spring Open Circuit Causing DTC B1787-00

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Vehicle: 2017 BYD Qin EV300. Symptom: Airbag warning light on steady. Retrieved current DTC B1787-00 using VDS2000. Diagnostic procedure: Technician disconnected the battery negative terminal, waited 90 seconds, and removed the driver airbag module. Measured the airbag second-stage circuit resistance at 2.1Ω (normal). Further testing of the clock spring body-side harness revealed an open circuit in the second stage. Disassembled the clock spring and found the internal flat ribbon cable broken at the steering wheel. Solution: Replaced the driver-side clock spring assembly (part number: B2J-58200XX). After reassembly and clearing the fault codes, the airbag warning light went out and the fault cleared.
BYD DTC AI Analysis

Qin Pro: Loose connector under steering wheel causing intermittent warning

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Vehicle: BYD Qin Pro DM. Symptoms: SRS warning light intermittent; frequent on rough roads. Diagnostics: Retrieved historic DTC B1787-00. Diagnosis: Yellow 2-pin airbag connector under steering wheel had not fully latched; harness pulled loose with gentle tug. No terminal oxidation, but contact surfaces showed slight burn marks. Repair: Cleaned terminals with electronic contact cleaner. Reconnected until latch clicked. Secured harness with cable ties. Cleared DTCs. Monitored one week with no recurrence.
BYD DTC AI Analysis

Water damage corroded the floor wiring harness, causing an open circuit in the airbag circuit.

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Vehicle: BYD Qin 100. Symptoms: Airbag warning light stayed on after flood damage repairs, storing DTC B1787-00. Diagnosis: Airbag and clock spring both tested normal. Traced the wiring harness and found the floor harness-to-steering column harness connector (under the driver-side dashboard) had standing water and green corrosion inside. The second stage circuit pin (typically the specific pin in the yellow wire) had corroded and broken. Repair: Cut out the corroded connector and soldered the harness to fix the break. Sealed the repair with waterproof insulation. Replaced the floor harness section if necessary. Cleared the fault code after repair; system returned to normal.
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.