NO
AU
B1787

DTC B1787 indicates an open circuit fault in the second-stage deployment circuit of the driver-side dual-stage front airbag — Atto 3

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.

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Cases Logged
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 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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.