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 — Seal U

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 — Seal U
  • 1Dedicated yellow airbag connector under the seat (usually located on the inner side of the seat track) is loose, oxidized, or has backed-out pins, causing poor contact or an open circuit.
  • 2Internal open circuit in the seat belt pretensioner assembly, typically an open igniter circuit or burnt internal coil. Common causes include water ingress, failure to replace a deployed pretensioner, or end of service life.
  • 3Long-term bending and wear caused wire breakage in the pretensioner wiring harness at the transition area between the seat frame and vehicle body (inside the outer seat trim panel or floor harness corrugated conduit).
  • 4The driver seat Occupant Classification Sensor (OCS) shares a wiring harness connector with the pretensioner (on some models); 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, backed-out pins, or deformed pins.
  • 3
    Resistance measurement: Disconnect the ACU connector (to prevent module damage). Measure the resistance between the pretensioner plug terminals using a multimeter. Standard value: 2.0-3.0Ω. If the reading is OL (open circuit) or <1Ω (short circuit), further inspect the wiring harness or replace the pretensioner.
  • 4
    Harness continuity test: Inspect the corrugated harness conduit on the outer side of the seat rail and at the floor pass-through. Peel back the conduit and check for wire fatigue fractures caused by frequent forward and backward seat movement. Repair the harness if necessary and rewrap 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's seat belt assembly (including the pretensioner mechanism).
  • 6
    System reset: Reconnect all connectors and restore battery power. Use the BYD dedicated diagnostic tool (VDS) to clear the 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 Seal U 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]