AU
B1786

DTC B1786 indicates the airbag control module (SRS ECU) detects the ignition circuit resistance of the driver-side dual-stage front airbag Stage 2 inflator exceeds the standard threshold (normal range: 2 — Atto 8

Safety System

DTC B1786 indicates the airbag control module (SRS ECU) detects the ignition circuit resistance of the driver-side dual-stage front airbag Stage 2 inflator exceeds the standard threshold (normal range: 2.0-3.0 ohms; fault threshold: >6 ohms or open circuit).

The Stage 2 airbag operates with the Stage 1 airbag during severe collisions, optimizing protection through delayed ignition or increased inflation.

High resistance indicates a high-resistance condition or an open circuit.

This prevents the Stage 2 airbag from deploying normally during a severe collision, reducing driver protection.

It typically does not affect Stage 1 airbag operation.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Broken internal wire or excessive contact resistance in the clock spring (spiral cable) second-stage circuit. Frequent steering wheel rotation commonly causes fatigue failure, or driving through water oxidizes the contacts.
  • 2Internal open circuit or aging failure of the driver airbag module second-stage squib, causing abnormally high resistance.
  • 3Airbag wiring harness connector (yellow waterproof plug) loose, terminals backed out, oxidized, corroded, or water ingress, causing poor contact.
  • 4Hidden open circuit, poor crimping, or short circuit due to wear in the dashboard wiring harness between the SRS ECU and the clock spring.
  • 5Airbag replaced after deploying in a collision, but the new airbag module model does not match, the connector is not fully seated, or the wiring harness repair is incorrect.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes for the SRS capacitor to fully discharge to prevent accidental airbag deployment during repair.
  • 2
    Fault Confirmation: Use the BYD dedicated diagnostic tool (VDS2100/EDS) to read the fault code. Confirm B1786 is a Current DTC rather than a History DTC, and record freeze frame data.
  • 3
    Visual inspection: Remove the driver-side airbag module (disconnect the clock spring connector first). Inspect the airbag connector (yellow) for looseness, water ingress, oxidation, or deformed pins. Inspect the clock spring exterior for burn marks.
  • 4
    Resistance measurement: Use a digital multimeter (low-current range) to measure the resistance of the airbag module second-stage igniter. Standard resistance is 2.0-3.0 ohms. A reading of infinity or >10 ohms indicates an internal fault in the airbag module.
  • 5
    Clock spring inspection: Turn the steering wheel to the left and right lock positions while measuring the resistance of the corresponding second-stage clock spring circuit. If the resistance fluctuates or is infinite, replace the clock spring assembly.
  • 6
    Harness continuity test: Measure harness continuity and insulation to ground between the SRS ECU connector and the clock spring connector to locate hidden open or short circuits.
  • 7
    Component replacement: Based on the inspection results, replace the faulty component (clock spring or airbag module) or repair the wiring harness. Use genuine BYD parts.
  • 8
    System reset: Reconnect all connectors, connect the battery, use the diagnostic tool to clear the fault code, and perform the SRS system self-test (turn the ignition switch ON and observe if the instrument cluster airbag light turns off after 6 seconds).
  • 9
    Verification test: Perform a simulated crash signal test (special equipment required) or a road test to confirm the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Tang DM clock spring oxidized after water ingress, causing DTC B1786

✓
A 2021 Tang DM developed DTC B1786 after driving through heavy rain and flood waters. Inspection found obvious water ingress on the driver-side carpet and below the steering wheel. Removing the clock spring revealed severe oxidation and blackening of the internal copper contacts. Measured the airbag second-stage circuit resistance at 8.7 ohms (high) using a multimeter; the reading fluctuated when turning the steering wheel. Replaced the clock spring assembly and resistance returned to 2.4 ohms. Cleared the DTC and road-tested for one week with no recurrence.
BYD DTC AI Analysis

BYD Yuan EV: Poor connector contact after accident repair

✓
2019 Yuan EV logged DTC B1786 following front collision repairs. Service history showed the driver's airbag had been replaced. Inspection found the second-stage connector (yellow short plug) had not fully locked during airbag reinstallation, causing the waterproof seal to fail with contact resistance measuring 12 ohms. After reseating the connector and confirming an audible click, resistance dropped to 2.2 ohms and the fault cleared.
BYD DTC AI Analysis

Prolonged high-frequency use caused the Qin Pro clock spring to fracture.

✓
A 2020 BYD Qin Pro (rideshare vehicle, 180,000 km) frequently set DTC B1786, appearing intermittently. Resistance measured normal when stationary, but became infinite when turning the steering wheel to a specific angle (full right lock). Disassembling the clock spring revealed a fatigue fracture in the second-stage circuit of the internal flat cable at the winding limit. Replaced the clock spring, eliminating the fault completely. Advised the owner to avoid holding the steering wheel at full lock for extended periods.
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.