AU
B1647

DTC B1647 indicates the driver-side seat belt pretensioner circuit resistance exceeds the upper limit set by the SRS control unit (standard value typically 2 — Seal U

Safety System

DTC B1647 indicates the driver-side seat belt pretensioner circuit resistance exceeds the upper limit set by the SRS control unit (standard value typically 2.0-3.0 Ω; refer to the vehicle workshop manual).

The seat belt pretensioner uses a pyrotechnic squib; its resistance reflects the electrical integrity of the firing element.

High resistance indicates an additional high-impedance point in the circuit.

Potential causes include an aging or broken internal pretensioner resistance wire, oxidized or corroded harness connectors, internal wire breaks, or poor contact.

This fault causes the SRS system to deem the pretensioner circuit unreliable.

During a collision, the pretensioner may fail to deploy, preventing the seat belt from retracting in time and severely compromising occupant protection.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Seat belt pretensioner internal aging: Internal resistance wire oxidizes and breaks due to time, temperature, or manufacturing defects, gradually increasing resistance.
  • 2Poor wiring harness connector contact: Oxidation, looseness, or water ingress corrosion increases contact resistance at the pretensioner connector below the driver-side B-pillar or under the seat.
  • 3Wiring harness mechanical damage: Frequent fore/aft seat adjustment or improper B-pillar trim removal/installation causes fatigue fractures in the internal copper strands (outer sheath intact but internal strands partially broken), resulting in high resistance.
  • 4Water ingress or moisture corrosion: After the vehicle drives through water, poor sealing of the pretensioner connector causes terminal oxidation, creating additional resistance.
  • 5Non-genuine parts or improper repair: Using an aftermarket pretensioner (resistance tolerance out of specification) or failing to fully seat the connector after previous accident repairs.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds for the SRS backup power supply to discharge completely to prevent accidental airbag deployment.
  • 2
    Visual inspection: Remove the driver's side B-pillar lower trim panel. Check the seat belt pretensioner wiring harness connector (usually located at the base of the B-pillar or under the seat) for looseness, oxidation, or signs of water ingress.
  • 3
    Resistance measurement: Use a dedicated SRS system tester to measure the resistance between the pretensioner connector terminals (do not use a standard multimeter resistance setting for direct measurement, as this may trigger the pretensioner). Compare the reading against the standard value (typically 2.0±0.1Ω or the 2.0-3.0Ω range).
  • 4
    Harness continuity test: Disconnect the SRS ECU connector. Measure harness continuity between the ECU and the pretensioner connector. Check for high resistance or intermittent open circuits.
  • 5
    Cross-check: Swap the driver-side and passenger-side pretensioner connectors (if applicable). Observe if the fault code transfers to confirm whether the issue is the pretensioner itself or the wiring harness.
  • 6
    Component replacement: If the pretensioner unit resistance exceeds the specified limit, replace the driver seat belt pretensioner assembly with an OEM part. If the wiring harness is faulty, repair or replace the wiring harness.
  • 7
    System reset: Reconnect all connectors, connect the battery, use the diagnostic tool to clear the fault code, and perform the 'SRS System Configuration' or 'Crash Detection Sensor Calibration' procedure.
  • 8
    Function check: Start the vehicle and confirm the airbag warning light turns off after the self-check. Adjust the seat position multiple times and verify the fault code does not recur.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro airbag warning light stays on

✓
After starting the vehicle, the airbag warning light on the instrument cluster stayed on. Retrieved fault code B1647-00 (driver's seatbelt pretensioner resistance too high) from the SRS system using a VDS diagnostic tool. Checked the vehicle history and found no collision history. Removed the B-pillar trim and inspected the pretensioner connector; the connector was secure and not loose. Measured the pretensioner resistance with a specialized tester; the reading was 4.8Ω, significantly higher than the standard 2.3Ω. Checked the wiring harness continuity between the SRS ECU and pretensioner; continuity was normal with no shorts or open circuits. Determined that internal resistance wire aging in the pretensioner caused the resistance drift. Replaced the genuine driver's seatbelt pretensioner assembly, cleared the fault code, and completed the system self-test. Fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM intermittent seat belt pre-tensioner fault

✓
Owner reported that the instrument cluster intermittently displayed “Check SRS System”. DTC B1647 and other related codes appeared alternately. Diagnosis found the fault readily occurred after adjusting the driver seat. Inspected the wiring harness beneath the seat and found the pretensioner harness at the seat rail mounting point had insulation damage from prolonged chafing, with internal copper wires partially fractured creating high-resistance contact. Further measurements showed normal resistance when the seat was stationary, but resistance fluctuated and exceeded the threshold during seat movement. Repaired the damaged harness, then rerouted and properly secured it to avoid interference with the seat rails, completely eliminating the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song Pro B-pillar connector oxidation caused high resistance

✓
The airbag warning light illuminated after heavy summer rain. A diagnostic scan revealed DTC B1647. Inspection found clear water ingress at the pretensioner harness connector below the driver-side B-pillar, with green oxidation on the connector pins. After cleaning with electrical contact cleaner and applying conductive protectant, resistance remained slightly above specification. Further inspection showed insufficient spring tension in the connector terminals, causing unstable contact resistance. Replaced the pretensioner harness connector (repair connector); resistance returned to normal. Cleared DTC, system operates normally. Check sunroof drain tubes for blockages—a blockage may be allowing water into the B-pillar.
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]