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 8

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 8
  • 1The pretensioner wiring harness connector (yellow connector) under the driver's seat is loose, has a backed-out pin, or has poor contact. This usually results from wear during fore/aft seat movement or failing to fully seat the connector after removing the seat.
  • 2Seat belt pretensioner internal resistance wire open circuit or resistance drift beyond tolerance (>5Ω or <1Ω). Common causes include pretensioner aging, moisture ingress, or internal pyrotechnic charge deterioration.
  • 3Oxidized or corroded pins, or broken wires at the seat wiring harness to body floor wiring harness connector (usually located on the inner side of the seat slide rail), commonly resulting from vehicle wading or prolonged use in damp environments.
  • 4SRS ECU internal detection circuit fault or software error causes incorrect identification of the pretensioner status. Swap with another seat belt pretensioner to confirm.
  • 5Failure to reconnect the pretensioner connector after seat removal during vehicle modification or repair, or installing a non-genuine 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 fault (Active), not a historical fault. Record the ambient temperature and vehicle status from the freeze frame data.
  • 2
    Disconnect the negative battery terminal. Wait at least 3 minutes for the SRS system capacitor to fully discharge. Remove the driver's seat (retain the 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 plug terminals (use a special probe to avoid damaging the pins). The standard value is 2.0-3.0 Ω. If the resistance is infinite (OL) or 0 Ω, the pretensioner has an internal open or short circuit. Replace the driver seat belt assembly.
  • 4
    If the pretensioner unit is normal, measure the wiring harness continuity between the pretensioner connector and the SRS ECU (usually 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 the specified conductive grease; if the wiring harness is damaged, repair it with waterproof tape or replace the wiring harness; if the pretensioner is faulty, install an OEM driver seat belt assembly (Note: the pretensioner is a single-use component; do not dismantle it for repair).
  • 6
    After completing the repair, reconnect all connectors and restore the battery power supply. Use the diagnostic tool to clear the fault codes, execute 'SRS System Self-check' and 'Configuration Parameter Read', and confirm no fault codes remain. Perform a simulated crash test (using the dedicated resistor load tool) and monitor the data stream to verify the pretensioner resistance value 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 8 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.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.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.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.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.
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]