AU
B1786-00

This fault code indicates the resistance of the driver's front airbag stage 2 (high-power deployment stage) squib circuit exceeds the normal threshold set by the SRS ECU (standard value is typically 1 — Atto 3

Safety System

This fault code indicates the resistance of the driver's front airbag stage 2 (high-power deployment stage) squib circuit exceeds the normal threshold set by the SRS ECU (standard value is typically 1.0–2.0 Ω; threshold is generally 2.5 Ω).

The dual-stage airbag system triggers in stages based on collision severity, with stage 2 providing greater inflation power during a severe collision.

Excessive resistance indicates high resistance in the squib circuit, which may cause the ECU to detect an open-circuit risk.

This prevents or delays stage 2 deployment during a severe accident, significantly reducing driver protection.

This is a hard fault that does not clear automatically and illuminates the airbag warning light on the instrument cluster.

The B1786-00 fault code on the Atto 3 indicates high resistance in the driver’s front airbag stage 2 squib circuit. This is a medium-severity SRS fault that can affect full airbag deployment in a severe crash and usually turns on the airbag warning light. Common causes include a worn clock spring or a faulty driver airbag module igniter. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Wear, breakage, or poor contact of the internal conductive slip ring in the clock spring (spiral cable), increasing circuit series resistance (most common)
  • 2Aging, moisture ingress, or a manufacturing defect in the internal second-stage igniter of the driver airbag module, causing an abnormal increase in intrinsic resistance.
  • 3Airbag wiring harness connector (yellow plug under the steering wheel) pins oxidized, loose, backed out, or showing green copper corrosion, causing additional contact resistance.
  • 4Vehicle water ingress, incomplete drying after water exposure, or prolonged exposure to high humidity causes micro-corrosion inside the connector, forming a high-resistance oxide layer.
  • 5SRS ECU internal sampling resistor or A/D conversion circuit fault causing inaccurate resistance detection (rare case after elimination).
What To Do
  • 1
    Safe power-down: Turn off the ignition switch, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use the VDS2000/ED400 diagnostic tool to read the fault codes. Confirm B1786-00 is a current fault (Active). Clear the fault code and read the codes again to confirm the fault returns.
  • 3
    Visual inspection: Check the yellow airbag wiring harness connector below the steering wheel (usually located beneath the combination switch) for looseness, water ingress, oxidized pins, or harness damage. If necessary, clean with electrical contact cleaner and apply special conductive grease.
  • 4
    Remove and inspect the airbag module: Remove the driver airbag (use a T30 or special tool; observe anti-static precautions). Use a digital multimeter to measure the resistance between the second-stage squib pins inside the airbag connector. Normal resistance is 1.0-2.0 Ω. A reading >2.5 Ω indicates an internal module fault.
  • 5
    Clock spring test: Disconnect the lower end of the clock spring (spiral cable) from the body wiring harness. Measure continuity between the corresponding terminals at the clock spring upper end (airbag side) and lower end (ECU side). Resistance must be <1Ω. Turn the steering wheel; resistance must remain stable. If resistance fluctuates or is >2Ω, replace the clock spring.
  • 6
    Harness continuity check: Measure harness continuity between the SRS ECU and the clock spring (<1 Ω) and insulation to ground/power supply (>10 MΩ) to rule out hidden internal open circuits or harness wear.
  • 7
    Component Replacement and Verification: Based on the measurement results, replace the faulty component (replacement priority: clock spring > airbag module > wiring harness). Reconnect the battery. Use the diagnostic tool to clear the fault codes and perform the 'SRS system self-check' or 'igniter resistance learning' function. Confirm no fault codes remain and the airbag warning lamp turns off normally.
Real-World Cases
BYD DTC AI Analysis

Internal fracture in the Qin EV300 clock spring caused high resistance.

✓
The airbag warning light on the instrument panel stayed on. Retrieved DTC B1786-00. Measured the airbag module resistance; readings normal. However, when testing continuity between the upper and lower terminals of the clock spring, resistance fluctuated between 2.8-5.0Ω with intermittent open circuits when rotating the steering wheel. Removed the clock spring and found micro-fractures in the internal flat ribbon cable from long-term steering wear. Replaced the clock spring (part number: KJ-3472910). Resistance returned to normal and the fault cleared.
BYD DTC AI Analysis

Flood-damaged vehicle with oxidised and corroded airbag connector

✓
Qin 80 owner reported the airbag warning light illuminated after driving through standing water in heavy rain. The diagnostic tool displayed DTC B1786-00. Inspection revealed water staining and green copper corrosion inside the yellow connector beneath the steering wheel. Oxidation on the pin surfaces caused contact resistance of 3.2Ω. Cleaned the connector with precision electronics cleaner, polished the pins with fine sandpaper, applied conductive protective grease and reconnected. Resistance returned to normal and the fault code did not return after clearing.
BYD DTC AI Analysis

Internal Stage 2 squib in airbag module degraded

✓
2017 Qin 100, 80,000 km. SRS warning light suddenly came on. Disconnected the airbag module and measured second-stage igniter resistance at 4.1 Ω — out of spec. Determined the internal igniter pyrotechnic charge had absorbed moisture or the pins had oxidized, increasing resistance. Replaced the driver airbag assembly (gas generator and igniter). Resistance returned to 1.4 Ω and the system passed the self-test.
BYD DTC AI Analysis

Loose wiring harness connection causing intermittent high resistance

✓
Airbag warning light illuminated intermittently, sometimes clearing after restart. Diagnosis revealed intermittent DTC B1786-00. Inspection found the internal locking tab on the clock spring-to-body harness connector loose, preventing the connector from seating fully; vehicle vibration caused contact resistance to fluctuate. Re-secured the wiring harness clip to ensure the connector locked completely. Fault has not recurred.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 overview →
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.