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 — Seal U

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 — Seal U
  • 1Pretensioner connector water ingress or moisture: Water on the driver-side floor from wading, car wash fluid ingress, or A/C condensate leakage causes a short circuit between connector terminals, reducing circuit resistance.
  • 2Harness insulation damaged and shorted to ground: Frequent forward and backward seat adjustment rubs the pretensioner harness (usually routed along the seat rail) against the metal bracket. The damaged insulation shorts the wire to the vehicle body ground.
  • 3Connector terminal deformation or oxidation: Loose plugs, bent pins, or electrolytic corrosion cause abnormal conduction between terminals, creating 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-off procedure: Switch 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 seat (the Qin series usually requires removing 4 retaining bolts) and find the yellow pretensioner connector under the seat (typically a 2-pin connector marked with an SRS label).
  • 3
    Visual inspection: Check that 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. If the resistance is less than 1.5 Ω, it 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, unwrap the wiring harness section by section to check for damage. Focus on the bends under the seat.
  • 6
    Connector handling: If water ingress is present, clean the terminals with electronic cleaner, blow them 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. Perform the 'SRS system self-check' function. Verify 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 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]