AU
B1785

This fault code indicates the driver-side dual-stage airbag Stage 2 squib circuit resistance falls below the standard threshold (typically <1 — Atto 8

Safety System

This fault code indicates the driver-side dual-stage airbag Stage 2 squib circuit resistance falls below the standard threshold (typically <1.0Ω; standard value 2.0-3.0Ω).

Electrically, this indicates a short circuit in the airbag ignition circuit (short to ground or wire-to-wire short).

The second stage deploys based on collision severity.

Low resistance causes the SRS ECU to detect a circuit fault and disable airbag deployment, or in extreme cases, abnormal current triggers unintended deployment.

This hard fault in the active safety system triggers an airbag self-test failure, disables the entire front-row frontal airbag protection function, and poses a severe safety hazard.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Airbag module internal squib short circuit: Stage 2 gas generator squib inside the driver airbag assembly internally short-circuited due to moisture ingress, aging, or manufacturing defects.
  • 2Internal short circuit in the clock spring (spiral cable): Long-term rotation wears or breaks the clock spring coil below the steering wheel, causing the wire end to short to ground, or the internal slip ring short-circuits.
  • 3Harness worn and shorted to ground: The harness from the SRS ECU to the steering wheel rubs against the steering column, instrument panel frame, or similar components, damaging the insulation and contacting vehicle body metal.
  • 4Connector water ingress or corrosion: Poor sealing allows water to enter the airbag wiring harness connector (usually located under the steering wheel or on the back of the airbag), causing a short circuit between terminals or a short to ground.
  • 5SRS ECU internal detection circuit fault: Airbag control unit internal sampling resistor or A/D conversion circuit fault causing inaccurate resistance detection (relatively uncommon).
What To Do
  • 1
    Safe power-down and wait: Disconnect the low-voltage battery negative terminal, wait at least 90 seconds (to fully discharge the SRS capacitor), and record the original fault code.
  • 2
    Visual inspection: Check the airbag wiring harness connector under the steering wheel and the clock spring connector for looseness, water ingress, corrosion, or burn marks. Check the wiring harness around the steering column for wear.
  • 3
    Airbag module isolation test: Remove the driver airbag, disconnect the connector, and use a digital multimeter to measure the resistance between the stage 2 ignition pins on the airbag plug side (normal: 2.0-3.0 Ω). If the resistance is <1.0 Ω, replace the airbag assembly.
  • 4
    Harness continuity check: Disconnect the connector between the SRS ECU and the clock spring. Measure the harness resistance from the ECU side to the clock spring input side (should be <1Ω). Measure the resistance of each of the two wires to body ground (should be >1MΩ). If the resistance to ground is too low, inspect the harness for damage.
  • 5
    Clock spring verification: Remove the clock spring. Measure continuity between the corresponding input and output pins, and measure insulation between each pin and the housing. If the second-stage circuit has a short or poor insulation, replace the clock spring.
  • 6
    ECU confirmation: If all previous checks are normal, install a known-good airbag module and clock spring for testing. If the fault code persists, the SRS ECU is faulty. Replace the control unit.
  • 7
    Repair and Verification: After replacing the faulty component, reconnect all connectors and restore power. Use VDS or the OEM diagnostic tool to clear the fault code. Perform an SRS system self-check (Active Test). Verify B1785 does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Internal short in clock spring caused B1785

✓
2020 BYD Qin Pro, 80,000 km. Owner reported the airbag warning light illuminating intermittently when turning the steering wheel to specific angles. VDS scan showed DTC B1785 (current fault). Disassembly revealed the clock spring's second-stage airbag flat cable worn through at rotational limit positions; exposed copper contacted the metal housing, causing a short to ground. Replaced the clock spring; new component measured 2.3Ω. Cleared fault codes after installation and performed multiple full-lock steering tests. Fault did not reoccur.
BYD DTC AI Analysis

Moisture ingress caused internal short circuit in airbag module

✓
A 2019 BYD Yuan EV had water leaking through the front windshield into the driver's side A-pillar and the airbag warning light constantly on. B1785 showed as a current fault and would not clear. Disconnected the airbag connector; the harness side measured infinite resistance to ground (normal), but the second-stage ignition pins on the module measured only 0.3Ω—indicating an internal short in the igniter. Removed the airbag and found the gas generator seal had deteriorated, allowing water in. Replaced the driver airbag assembly and repaired the windshield seal. Fault resolved.
BYD DTC AI Analysis

Steering wheel modification damaged the wiring harness

✓
2021 BYD Tang DM. The owner installed a racing steering wheel and later restored the factory unit, triggering the SRS warning light. DTC B1785 indicated resistance below specification (0.8Ω). Inspection revealed the modification had overstretched the airbag wiring harness, deforming terminals inside the connector below the steering column. The second-stage airbag circuit terminal shorted to an adjacent terminal. Repaired the connector terminals, rerouted the wiring with adequate slack, and restored normal resistance (2.1Ω). Fault resolved.
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.