AU
B1647-00

This fault code indicates the airbag control unit (SRS ECU) detects the driver-side seat belt pretensioner circuit resistance exceeds the standard range (normal: 2 — Seal U

Safety System

This fault code indicates the airbag control unit (SRS ECU) detects the driver-side seat belt pretensioner circuit resistance exceeds the standard range (normal: 2.0-3.0Ω; high resistance generally indicates >5Ω or a near open circuit).

The pretensioner contains an electrothermal igniter (squib).

The ECU continuously monitors circuit resistance using a low-current signal to verify continuity.

High resistance typically indicates high impedance or an open circuit.

This condition may prevent the pretensioner from deploying and retracting the seat belt during a collision, severely compromising occupant protection.

The SRS system enters fail-safe mode, illuminates the instrument cluster airbag warning light, and may disable the front airbags on certain models.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Pretensioner connector poor contact or oxidation: Frequent vibration or moisture causes terminal oxidation, terminal back-out, or looseness in the dedicated yellow connector located at the bottom of the B-pillar or under the seat, creating contact resistance.
  • 2Pretensioner internal open circuit: The pretensioner igniter (squib) has an internal open circuit due to aging, moisture ingress, or a previous abnormal voltage impact, resulting in infinite resistance.
  • 3Wiring harness wear or breakage: Long-term seat adjustment wears the harness insulation near the seat rail, or pinching the harness during B-pillar trim panel removal and installation breaks the copper core.
  • 4Airbag ECU internal fault: A damaged sampling resistor or A/D conversion module in the SRS control unit internal detection circuit causes a false high resistance reading (relatively uncommon).
  • 5Interference from modified or added equipment: Cutting into wires during unauthorized installation of seat heating, massage functions, or B-pillar speakers damages the shielding and integrity of the pretensioner circuit.
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS backup capacitor. Wear an anti-static wrist strap. Never measure the pretensioner directly using a multimeter resistance setting. Use a dedicated diagnostic tool or high-impedance test equipment.
  • 2
    Visual inspection: Check that the yellow dedicated SRS connector under the B-pillar (or under the seat) is fully locked. Inspect the terminals for green oxidation or burn marks, and check the wiring harness sleeve for damage.
  • 3
    Measurement verification: Use the diagnostic tool to read the live data stream and confirm the specific resistance value. Disconnect the pretensioner connector and short the ECU-side connector using a dedicated jumper wire. If the fault code changes to "resistance too low", the wiring harness from the ECU to the connector is normal; the fault lies in the pretensioner itself or the connector contact.
  • 4
    Step-by-step diagnosis: If the wiring harness is normal, use the special tool to measure the pretensioner body resistance (specification: 2-3Ω). If the resistance is infinite, replace the pretensioner assembly. If the resistance is normal, clean the connector terminals and apply special conductive grease.
  • 5
    Reset verification: Reconnect all connectors, reconnect the battery, and use VDS or a dedicated diagnostic tool to clear the fault code. Perform an SRS system self-check (typically, the warning light illuminates for 6 seconds after turning the ignition switch ON, then turns off). Finally, perform a simulated crash test (trigger using a dedicated tool, not an actual vehicle crash) to verify normal communication in the pretensioner circuit.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 B-pillar wiring harness wear causing intermittent fault

✓
The airbag warning light illuminated intermittently on the dash. Retrieved DTC B1647-00; resistance measured 8.5Ω (high). Inspection found the wiring conduit between the driver's seat rail and B-pillar trim damaged, exposing a twisted pair with one wire nearly broken. The harness chafed against a metal bracket from the owner's frequent seat height adjustments. Repaired the wiring, rerouted and secured the harness, and fitted abrasion-resistant sleeving. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin 100 pretensioner connector oxidised after water ingress

✓
After driving through water, the airbag warning light stayed on. The scan tool showed code B1647-00. Removing the B-pillar lower trim panel revealed obvious water staining and copper oxide inside the yellow pretensioner connector, with the terminals blackened. The Qin series B-pillar lower seals deteriorate with age and allow water ingress, so moisture entered the connector and caused high contact resistance (measured 5.8Ω). Cleaned the terminals, sprayed precision instrument cleaner, applied conductive grease, and resealed. Cleared the fault code and it has not returned.
BYD DTC AI AnalysisFrom Chinese market (translated)

Pretensioner not replaced after accident repair caused open circuit.

✓
Following a front collision, a non-authorised workshop repaired the vehicle, replacing the front bumper but failing to address the seatbelt pretensioner. Three months later, DTC B1647-00 set. The pretensioner resistance measured infinite. Disassembly revealed the pretensioner had deployed during the accident (internal pyrotechnic charge had fired but mechanical structure showed no visible deformation), indicating a latent failure. Replaced the driver seatbelt retractor assembly (including pretensioner) and performed online configuration to write new ECU calibration data, resolving the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Mistakenly modified SRS wiring while retrofitting seat ventilation

✓
The owner had a seat ventilation system installed at an aftermarket shop. While tapping into a wire for power, the technician accidentally cut the pretensioner return harness and reconnected it with a twisted splice, causing poor contact and increased resistance (4.2Ω). Because pretensioner wiring uses twisted-pair shielded cable, ordinary splicing methods compromised the circuit characteristics and caused unstable resistance. We restored the original harness, crimped OEM butt connectors, and re-did the shield grounding to resolve 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]