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 — Qin Plus

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 Qin Plus indicates high resistance in the driver’s front airbag stage 2 squib circuit, which can prevent or delay full airbag deployment in a severe crash. This is a medium-severity SRS fault and usually turns on the airbag warning light. Common causes include a worn clock spring or a faulty driver airbag 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 — Qin Plus
  • 1Internal conductive slip ring in the clock spring (spiral cable) worn, broken, or making poor contact, increasing circuit series resistance (most common)
  • 2Aging, moisture ingress, or a manufacturing defect in the internal second-stage igniter of the driver airbag module causes an abnormal increase in intrinsic resistance.
  • 3Oxidized, loose, or backed-out pins, or green copper corrosion at the airbag wiring harness connector (yellow plug under the steering wheel), creating additional contact resistance.
  • 4Water ingress, incomplete drying after water exposure, or a prolonged high-humidity environment 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, diagnosed by 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 code and confirm B1786-00 is a current fault (Active). Clear the code, then read it again to confirm the fault reoccurs.
  • 3
    Visual inspection: Inspect 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 Stage 2 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 upper end (airbag side) and lower end (ECU side) of the clock spring. Resistance must be <1Ω. Rotate 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 (>10 MΩ) to rule out hidden internal open circuits or wear.
  • 7
    Component Replacement and Verification: Replace the faulty component based on the measurement results (replacement priority: clock spring > airbag module > wiring harness). Reconnect the battery. Use the diagnostic tool to clear the fault code 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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.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.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.