AU
B1785-00

This fault code indicates the circuit resistance of the driver-side dual-stage front airbag second-stage igniter (squib) falls below the threshold set by the SRS control unit (standard value: 2 — Seal U

Safety System

This fault code indicates the circuit resistance of the driver-side dual-stage front airbag second-stage igniter (squib) falls below the threshold set by the SRS control unit (standard value: 2.0 ± 0.3 Ω; fault triggers at <1.0 Ω or near 0 Ω).

The second-stage airbag deploys with a delay during severe collisions, providing staged protection alongside the first stage.

Low resistance indicates a short circuit risk, which may cause: 1) the second-stage airbag to fail to deploy normally during a collision, reducing protection effectiveness; 2) unintended airbag deployment in extreme cases due to wiring abnormalities.

This is a hard fault.

The SRS system illuminates the warning light and may disable related airbag functions.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Driver airbag module internal second-stage igniter shorted or damaged (airbag assembly fault)
  • 2Clock spring (spiral cable/clockspring reel) internal short circuit or damaged interlayer insulation
  • 3Water ingress, corrosion, or bent pins in the airbag wiring harness connector causing abnormal continuity.
  • 4Fault in the SRS control unit internal sampling resistor or A/D conversion circuit causing a false alarm.
  • 5Wiring harness pinched or damaged during steering wheel modification or removal/installation, causing a short to ground.
What To Do
  • 1
    Use a dedicated diagnostic tool (e.g., BYD ED400 or Launch X431) to read the fault code. Confirm B1785-00 is a current fault, not a history fault, and record the freeze frame data.
  • 2
    Visually inspect the steering wheel and airbag cover for damage, signs of water ingress, or unauthorized modifications. Check the SRS fault indicator status.
  • 3
    Disconnect the battery negative terminal and wait at least 90 seconds (to completely discharge the SRS backup capacitor). Do not omit this step.
  • 4
    Remove the driver airbag module (use a special tool or Allen wrench; avoid forceful removal) and check the clock spring connector for water ingress, oxidation, or deformed pins.
  • 5
    Replace the actual airbag with an airbag simulator (2Ω substitute resistor). Measure the circuit resistance from the clock spring to the SRS ECU. Normal resistance is 2.0-3.0Ω. A resistance <1.5Ω indicates a short circuit in the wiring.
  • 6
    Sectional inspection: Disconnect the clock spring and measure the airbag side and wiring harness side separately. The airbag side resistance must be >10kΩ or infinite; a lower value indicates an internal airbag short circuit. The wiring harness side resistance must be 2-3Ω; a lower value indicates a wiring harness short circuit.
  • 7
    Check the SRS ECU connector (usually located under the center console or near the gear selector) for water ingress and corrosion. Measure the insulation resistance to ground of the ECU-side wiring harness (should be >1 MΩ).
  • 8
    Replace the faulty component based on the measurement results: if the airbag resistance is abnormal, replace the airbag assembly; if the clock spring is abnormal, replace the clock spring; if the wiring harness is abnormal, repair the wiring harness.
  • 9
    Reinstall all components, connect the battery, clear the fault code, and perform a system self-check (ignition switch in the ON position: SRS warning lamp illuminates for 6 seconds then turns off, indicating normal operation). If necessary, perform an on-vehicle collision simulation test (using special equipment).
Real-World Cases
BYD DTC AI Analysis

2018 BYD Qin EV450 clock spring internal short causes low resistance

✓
After driving through water, the SRS warning light stayed on. Retrieved DTC B1785-00. Disassembly revealed water inside the clock spring connector. Airbag assembly resistance measured normal (2.1Ω), but the clock spring output terminal showed only 0.8Ω to ground. Replaced the clock spring; fault cleared. Root cause: water created conductive paths between the clock spring layers, pulling down the secondary airbag circuit resistance.
BYD DTC AI Analysis

Aftermarket steering wheel modification damaged wiring harness on 2017 Qin 80, causing short circuit

✓
The owner fitted an aftermarket racing steering wheel but later reverted to the factory unit. DTC B1785-00 appeared shortly after. Inspection revealed the steering wheel frame's sharp edge had cut the airbag wiring harness during removal and refitting, causing copper wires to contact the metal frame and short to ground. Repaired the harness and re-insulated the damaged section, restoring resistance to 2.3Ω. Cleared the fault code and the system returned to normal.
BYD DTC AI Analysis

Qin EV300 airbag module internal igniter fault

✓
The SRS warning light suddenly illuminated during normal driving. No water ingress or collision history. Measured second-stage circuit resistance at the airbag connector: only 0.3Ω, significantly below spec. Disconnected the clock spring and measured airbag resistance directly: still 0.3Ω. Diagnosed as internal short in the second-stage igniter. Replaced the driver airbag assembly (including airbag module and igniter). Fault resolved.
BYD DTC AI Analysis

SRS ECU sampling circuit fault — false positive

✓
Measured airbag and wiring harness resistance multiple times; readings normal (2.2Ω). ECU still set B1785-00. Oscilloscope check of ECU sampling terminal revealed abnormal pulse interference. Power and ground good. Fault isolated to internal ECU analogue front-end circuit. Replaced SRS control unit, recoded and matched. Fault did not recur.
Other Seal U 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.