AU
B1640

DTC B1640 indicates the SRS (Supplemental Restraint System) ECU detects an open circuit or out-of-range resistance in the driver-side seat belt pretensioner circuit (normal resistance is typically 2 — Atto 3

Safety System

DTC B1640 indicates the SRS (Supplemental Restraint System) ECU detects an open circuit or out-of-range resistance in the driver-side seat belt pretensioner circuit (normal resistance is typically 2.0–3.0 Ω).

The pretensioner is a key component of the passive safety system.

During a collision, it ignites a pyrotechnic charge to generate gas, pulling the seat belt retractor and instantly tightening the seat belt to secure the occupant.

This fault indicates the driver-side pretensioner may fail to deploy during a collision, increasing occupant forward displacement and injury risk.

The SRS ECU also illuminates the instrument cluster airbag warning light.

The system may enter a fail-safe mode, restricting certain airbag functions.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1The pretensioner wiring harness connector (yellow connector) under the driver's seat is loose, has a backed-out terminal, or makes poor contact, usually resulting from wear when moving the seat back and forth or failing to fully seat the connector after previous seat removal.
  • 2Open circuit or resistance drift beyond tolerance (>5Ω or <1Ω) in the seat belt pretensioner internal resistance wire. Pretensioner aging, moisture ingress, or internal pyrotechnic charge deterioration usually cause this.
  • 3Oxidized or corroded pins in the mating connector between the seat wiring harness and the body floor wiring harness (usually located on the inner side of the seat slide rail), or a broken wire in the harness. This commonly occurs after driving through water or prolonged use in damp environments.
  • 4An internal fault in the SRS ECU detection circuit or a software false positive causes incorrect identification of the pretensioner status. Confirm by swapping with another seat belt pretensioner.
  • 5Failure to reconnect the pretensioner connector after removing the seat during vehicle modification or repair, or installing a non-OEM seat resulting in a wiring harness mismatch.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000/VDS3000) to read the fault code. Confirm B1640 is a current (Active) fault, not a history fault. Record the ambient temperature and vehicle status from the freeze frame data.
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes for the SRS system capacitors to fully discharge. Remove the driver's seat (retain 4 bolts for easier handling). Visually inspect the yellow pretensioner connector under the seat to confirm it is fully inserted and locked. Verify the connector waterproof seal is intact.
  • 3
    Use a multimeter to measure the resistance between the pretensioner connector terminals (use a dedicated probe to avoid damaging the pins). The standard value is 2.0-3.0Ω. If the resistance is infinite (OL) or 0Ω, the pretensioner has an internal open or short circuit. Replace the driver seat belt assembly.
  • 4
    If the pretensioner is normal, measure wiring harness continuity between the pretensioner connector and the SRS ECU (usually located under the centre console or centre tunnel). Check for an open circuit (resistance >1Ω) or a short to ground/power. Inspect the harness bends near the seat slide rails for wear.
  • 5
    Inspection and repair: If the connector is loose, reconnect it and apply specified conductive grease; if the wiring harness is damaged, repair it with waterproof tape or replace the harness; if the pretensioner is faulty, replace the OEM driver seat belt assembly (Note: The pretensioner is a single-use component; do not dismantle or repair it).
  • 6
    After repair, reconnect all connectors and restore battery power. Use the diagnostic tool to clear fault codes. Execute 'SRS system self-check' and 'configuration parameter reading' to confirm no fault codes remain. Perform a simulated crash test (using the dedicated resistor load tool) and verify the pretensioner resistance value in the data stream is stable within the normal range.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Frequent seat adjustment loosened the pretensioner connector.

✓
A 2020 BYD Qin Pro EV driven 32,000 km had an illuminated airbag warning light. Scanned for DTCs and retrieved B1640 (current fault). Inspection found the yellow connector under the driver's seat inserted but the locking tab not fully engaged. The connector had gradually come loose because the owner frequently moves the seat forward and backward. Reseated the connector firmly and secured the harness routing. Cleared the DTC; fault resolved. Recommend securing the pretensioner harness to the seat frame with cable ties to prevent interference with the seat rails.
BYD DTC AI AnalysisFrom Chinese market (translated)

Water ingress corroded the pretensioner connector, causing high resistance.

✓
A 2019 BYD Yuan EV had water ingress from a blocked sunroof drain tube, soaking the floor wiring harness. DTC B1640 appeared intermittently. The pretensioner body measured 2.3 Ω (normal), but a 0.8 V voltage drop across the under-seat connector pins indicated high contact resistance. Disassembly revealed the internal pins had oxidised and turned black. Cleaning the pins, applying conductive grease, and replacing the waterproof seal resolved the fault. Also recommended checking the SRS ECU for moisture ingress.
BYD DTC AI AnalysisFrom Chinese market (translated)

Pretensioner circuit not reconnected after seat modification

✓
The airbag warning light illuminated on a 2021 BYD Tang DM after the owner swapped in racing seats. Diagnosis revealed DTC B1640, and live data showed the pretensioner circuit was open. Inspection found the aftermarket seats lacked pretensioners, and the original pretensioner wiring harness was wrapped in insulating tape and left dangling, disconnected. Solution: Reinstall the factory seat assembly, or install aftermarket seats with pretensioners and properly connect the circuit. A dedicated 2.7 Ω resistor can temporarily simulate the load and clear the fault code, but this is not recommended for long-term use.
BYD DTC AI AnalysisFrom Chinese market (translated)

Pretensioner internal resistance wire open circuit

✓
A 2019 Song MAX set DTC B1640 after a front collision repair. The collision deployed the front airbag but did not trigger the driver’s pretensioner. However, inspection revealed the pretensioner’s internal resistance wire had fractured (infinite resistance). A current surge during the accident or static damage from an earlier repair likely caused the failure. We replaced the driver seat belt assembly (including pretensioner) and performed ‘component replacement coding’ and ‘system calibration’ with a scan tool. The fault cleared. After replacement, verify the new assembly’s resistance is 2.0–3.0 Ω.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness wear caused an intermittent open circuit.

✓
On a 2021 BYD Yuan Pro, the airbag warning light would occasionally illuminate over rough roads. A scan retrieved stored fault code B1640. Static resistance of the pretensioner measured normal, but the reading fluctuated when the seat was shaken. Inspection found the wiring harness sheath chafed on the inner side of the seat rail; internal copper conductors were partially fractured and barely connected. The repair: cut out the damaged section, soldered in an extension wire, sealed with heat shrink tubing, and rerouted the wiring away from the seat rail travel path. The fault was permanently eliminated.
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]