AU
B1646-00

DTC B1646-00 indicates the driver-side seat belt pretensioner circuit resistance is below the standard threshold set by the SRS control unit (typically 2 — Atto 3

Safety System

DTC B1646-00 indicates the driver-side seat belt pretensioner circuit resistance is below the standard threshold set by the SRS control unit (typically 2.0Ω±0.1Ω).

The pretensioner is a pyrotechnic actuator containing an igniter pellet and a heating wire; normal resistance is 1.5-3.0Ω.

Low resistance typically indicates a short circuit (such as a short to ground, a wire-to-wire short, or an internal pretensioner short), reducing current path resistance.

This fault forces the SRS system into a degraded mode.

During a collision, the system may fail to deploy the driver-side pretensioner or, in extreme cases, cause unintended deployment, severely compromising passive safety system functionality.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Water ingress or moisture in the pretensioner connector: Water accumulation on the driver side floor, car wash fluid ingress, or an A/C condensate leak causes a short circuit between the connector terminals, reducing circuit resistance.
  • 2Harness insulation damaged and shorted to ground: Frequent forward and backward seat adjustment causes the pretensioner harness (usually routed along the seat rail) to rub against the metal bracket. Damaged insulation shorts the wire to the vehicle body.
  • 3Connector terminal deformation or oxidation: Loose plugs, bent pins, or electrolytic corrosion cause abnormal conduction between terminals, forming a parallel resistance path.
  • 4Pretensioner internal short circuit: Aged or damp igniter heating wire insulation causes an internal inter-turn short circuit (uncommon but requires inspection).
  • 5SRS control unit sampling circuit fault: Faulty internal ECU A/D converter or sampling resistor causes incorrect resistance readings (rule out via cross-checking).
What To Do
  • 1
    Safe power-down procedure: Turn off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Locate the pretensioner assembly: Remove the driver's seat (Qin series typically requires removing 4 retaining bolts) and find the yellow pretensioner connector under the seat (typically a 2-pin connector marked with the SRS label).
  • 3
    Visual inspection: Verify the connector sealing ring is intact. Inspect the terminals for green oxidation, burn marks, or liquid ingress. Check the wiring harness sleeve for wear at the seat rail.
  • 4
    Resistance measurement: Use a digital multimeter (0.1 Ω accuracy) to measure the resistance between the pretensioner-side terminals. The normal value is 1.8-2.2 Ω. A reading below 1.5 Ω indicates an internal short circuit; replace the seat belt assembly.
  • 5
    Insulation test: Measure the resistance between the pretensioner circuit and body ground. The resistance must be greater than 1 MΩ. If the resistance is too low, strip the wiring harness section by section to locate the damage. Focus on the bend under the seat.
  • 6
    Connector handling: If detecting water ingress, clean the terminals with electrical contact cleaner, blow dry, and apply conductive grease. Replace the connector housing if necessary (part number: BYD-SRS-CONN-01).
  • 7
    System reset: After repair, reconnect the battery. Use a diagnostic tool (VDS2000 or Launch X431) to clear the fault code. Execute the 'SRS system self-check' function. Confirm B1646-00 does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Water-Wading Vehicle Seatbelt Pretensioner Connector Short Circuit Repair Case

✓
A 2017 BYD Qin EV300 owner reported the airbag warning light staying on. Retrieved fault code B1646-00 (driver's pretensioner resistance too low). Inspection found the vehicle had previously been through water. The yellow pretensioner connector under the driver's seat contained visible moisture; resistance between terminals measured 0.8Ω (short circuit). Disassembling the connector revealed the seal had aged and failed. Solution: Replaced the waterproof connector assembly, used compressed air to blow dry the inside of the wiring harness, and applied silicone grease inside the connector to enhance sealing. After reset, the fault code cleared and resistance returned to 2.0Ω. Recommend checking the floor drain hole on that side for blockage to prevent future water ingress.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat rail chafing wiring harness causing intermittent short circuit

✓
2018 Qin EV450 with intermittent SRS warning light and DTC B1646-00. Found the fault appears when the seat is in the rearmost position and clears when moved forward. Disassembly revealed the pretensioner wiring harness (routed along the left seat rail) pinched at the seat track end, with insulation worn through exposing copper wire, causing a short to ground via contact with the seat frame. Repaired the damaged wire using double-layer heat-shrink insulation, rerouted the harness away from moving component interference, and secured with cable ties 10 mm above the seat frame to ensure no rubbing throughout full seat travel.
BYD DTC AI AnalysisFrom Chinese market (translated)

Modified seats caused poor connector contact

✓
A 2017 BYD Qin 100 triggered a warning light after the owner fitted racing seats. Inspection found the aftermarket workshop had cut off the original pretensioner plug rather than using a dedicated adapter harness, wrapped the connections with ordinary tape, causing a short circuit between terminals and unstable resistance (measured fluctuating between 0.3–1.2Ω). Repair: Sourced an identical plug from a wrecker to restore the original connector, soldered the wiring and sealed it with heat-shrink tubing, ensuring smooth solder joints without burrs. Post-repair resistance measured 1.9Ω and the fault cleared. Reminder: When modifying seats, use a high-resistance decoder or retain the original pretensioner circuit. Direct shorting is strictly prohibited.
BYD DTC AI AnalysisFrom Chinese market (translated)

Pretensioner internal igniter short-circuited due to aging.

✓
2017 Qin 80, 80,000 km, no accident history. DTC B1646-00 appeared suddenly. Checked connectors and wiring harness — both normal. Disconnected the plug and measured pretensioner resistance at only 0.5 Ω. Diagnosis: internal short between heating wire layers. The pretensioner is non-serviceable, so replaced the driver-side seatbelt assembly (including pretensioner). After replacement, use a diagnostic tool to run 'Configuration Setting' and enter the new seatbelt resistance code (usually on the label, range 1.5–3.0 Ω) so the SRS ECU correctly recognises the new component parameters.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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. Sources: [1]