AU
B1640-00

This DTC indicates the Airbag Control Unit (ACU) detects an open circuit or abnormal resistance in the driver-side seat belt pretensioner circuit — Atto 3

Safety System

This DTC indicates the Airbag Control Unit (ACU) detects an open circuit or abnormal resistance in the driver-side seat belt pretensioner circuit.

The pretensioner contains a squib and a resistance wire; normal resistance typically measures 2.0-3.0Ω.

The ACU triggers B1640-00 when it detects infinite resistance (open circuit) in the pretensioner circuit or when the resistance remains outside the threshold range for a specified time.

During a collision, this fault may prevent the driver-side seat belt pretensioner from deploying and tightening, severely compromising occupant restraint protection.

The fault also forces the SRS system into a degraded protection mode and continuously illuminates the airbag warning light.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Dedicated yellow airbag connector under the seat (usually located on the inner side of the seat rail) is loose, oxidized, or has recessed pins, causing poor contact or an open circuit.
  • 2Open circuit inside the seat belt pretensioner unit, usually an open igniter circuit or burnt internal coil. Common causes include water ingress, previous deployment without replacement, or end of service life.
  • 3Pretensioner wiring harness wire broken in the transition area between the seat frame and vehicle body (inside the outer seat trim panel or floor harness corrugated conduit) due to long-term bending and wear.
  • 4The driver seat occupancy sensor (OCS) and pretensioner share a wiring harness connector (depending on configuration). Poor connector contact consequently affects pretensioner circuit detection.
  • 5Airbag Control Unit (ACU) internal driver circuit fault or connector pin corrosion, causing abnormal detection voltage in the pretensioner circuit.
What To Do
  • 1
    Safe power-down: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS system capacitors and prevent accidental airbag deployment during repair.
  • 2
    Visual inspection: Verify the yellow dedicated connector under the driver's seat (marked AIRBAG or SRS) is fully locked. Inspect the inside of the connector for water stains, green corrosion, or backed-out or deformed pins.
  • 3
    Resistance measurement: Disconnect the ACU connector to prevent module damage. Use a multimeter to measure the resistance between the pretensioner plug terminals. The standard value is 2.0-3.0Ω. If the reading shows OL (open circuit) or <1Ω (short circuit), troubleshoot the wiring harness or replace the pretensioner.
  • 4
    Harness continuity test: Carefully inspect the wiring harness corrugated conduit on the outside of the seat rail and at the floor pass-through. Strip the conduit to check for wire fatigue breakage caused by frequent forward and backward seat movement. If necessary, repair the wiring harness and re-wrap with abrasion-resistant tape.
  • 5
    Replacement verification: If harness continuity is normal, perform a substitution test using a known-good pretensioner or seat belt assembly. If the fault code clears, the pretensioner is faulty. Replace the driver seat belt assembly (including the pretensioner mechanism).
  • 6
    System reset: Reconnect all connectors, restore battery power, and use the BYD dedicated diagnostic tool (VDS) to clear fault codes. Perform the SRS system self-diagnosis and pass the crash sensor simulation test. Confirm the fault light turns off and the system has no current fault codes.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat adjustment caused connector to loosen on Qin EV450

✓
A 2018 BYD Qin EV450 at 30,000 km came in with the SRS warning light on and DTC B1640-00 stored. The technician found that when the driver seat was moved fully rearward, a gap appeared between the yellow connector under the seat and the floor wiring harness. The connector lock was not fully engaged, causing an intermittent open circuit. Reseating the connector and securing the harness to the seat frame with cable ties to prevent it from moving with the seat fixed the fault. This issue is common in fleet vehicles where seats are frequently adjusted.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin 100 pretensioner internal open circuit fault

✓
2017 BYD Qin 100. After a low-speed collision, the SRS warning light stayed on with fault code B1640-00. Although the collision did not deploy the main airbag, inspection found infinite internal resistance in the driver's pretensioner. Disassembling the seatbelt retractor revealed the pretensioner squib had an internal open circuit due to a previous slight trigger (incomplete combustion). Replaced the driver's side seatbelt assembly, cleared fault codes with VDS, and the system passed the self-test. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness worn through at seat rail causing short circuit

✓
A Qin EV300 in commercial service logged an intermittent B1640-00 fault; the warning lamp occasionally went out on rough roads. The technician found the wiring harness inside the outer seat trim had been chafing against the metal seat rail, wearing through the insulation and causing an intermittent short to ground. The technician repaired the damaged harness (2 pretensioner wires), fitted abrasion-resistant corrugated tubing, and rerouted the harness to avoid interference with moving parts. This completely resolved the fault.
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]