AU
B176D

DTC B176D indicates the SRS (Supplemental Restraint System) ECU detects an open circuit in the left rear seat belt pretensioner circuit (infinite resistance), meaning the ECU cannot detect the pretensioner — Seal U

Safety System

DTC B176D indicates the SRS (Supplemental Restraint System) ECU detects an open circuit in the left rear seat belt pretensioner circuit (infinite resistance), meaning the ECU cannot detect the pretensioner.

This typically indicates a broken circuit connection between the pretensioner and the ECU, or an internal open circuit within the pretensioner.

As a key passive safety actuator, the pretensioner rapidly tightens the seat belt via pyrotechnic deployment or motor operation during a collision, removing slack between the webbing and the occupant.

This fault disables the left rear seat pretensioner during a collision, increasing occupant injury risk.

The ECU continuously monitors the pretensioner circuit resistance (normal range: approx. 2.0-3.0Ω).

It sets a 'not present' fault when it detects a resistance >10Ω or a complete open circuit.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Left rear seat belt pretensioner wiring harness connector loose or making poor contact (commonly occurs due to incomplete connector seating after rear seat removal, or a broken locking tab causing an intermittent connection)
  • 2Internal open circuit in the pretensioner unit (broken pretensioner squib wire or internal circuit board failure, preventing the ECU from establishing a complete circuit)
  • 3Physical damage to the wiring harness (long-term pinching by the seat slide rail, or the seat folding mechanism chafing the harness insulation, causing internal copper wires to break)
  • 4Connector terminal corrosion, oxidation, or back-out (liquid intrusion into the connector during rear area cleaning causes terminal oxidation and excessive contact resistance)
  • 5SRS ECU internal detection circuit fault (damaged ECU sampling resistor or detection chip falsely reports pretensioner missing; actual wiring is normal)
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS system capacitors and prevent accidental airbag deployment.
  • 2
    Initial visual inspection: Remove the left rear seat (usually requires removing the cushion retaining bolts). Verify the pretensioner connector (usually located inside the C-pillar trim panel or under the seat) is fully connected. Inspect the wiring harness for obvious damage or crush marks.
  • 3
    Connector reset and cleaning: Disconnect the pretensioner connector. Check the pins for oxidation, corrosion, or bending. Clean with electronic connector cleaner, then reconnect. Listen for a 'click' to confirm locking.
  • 4
    Resistance measurement verification: Use a multimeter to measure the resistance on the pretensioner side. The normal value is 2.1-2.9Ω (at 20°C). If the resistance is infinite, replace the pretensioner assembly.
  • 5
    Harness continuity check: Measure the resistance of the harness between the SRS ECU connector and the pretensioner connector; resistance must be less than 1 Ω. Measure the insulation resistance to ground; resistance must be greater than 1 MΩ. This rules out an open circuit in the harness or a short to ground.
  • 6
    Replacement verification (cross-test): Swap the left-rear and right-rear pretensioner connectors (if vehicle configurations match). Check if the fault code follows the component to isolate the fault to the pretensioner or the wiring harness/ECU.
  • 7
    ECU software check: Use VDS2000 or a dedicated BYD diagnostic tool to check the SRS ECU software version. Upgrade to the latest version if necessary to rule out software false alarms.
  • 8
    Repair verification: Reconnect the battery, clear the fault code, cycle the ignition switch to the ON position, and wait 20 seconds. Verify the SRS warning light turns off and no current B176D fault code is present. Perform a dynamic road test to verify.
Real-World Cases
BYD DTC AI Analysis

Left rear seat connector not reconnected after removal caused B176D

✓
Model: BYD Song MAX 2019. Owner reported the airbag warning light stays on. VDS scan retrieved DTC B176D-00 (Left rear seatbelt pretensioner does not exist), history fault. Owner had removed the left rear seat to install floor mats one week prior. Removed the seat and found the pretensioner connector (yellow plug) only half-inserted with the locking tab disengaged. Seat removal pulled the harness, loosening the connector. Reconnected the connector, confirmed the lock engaged, cleared the DTC, and the warning light went out. Road tested 20 km with no recurrence.
BYD DTC AI Analysis

Seat rail crushed wiring harness causing intermittent B176D fault

✓
Vehicle: BYD Qin Pro 2020. Customer reported the airbag warning light illuminated intermittently, particularly after adjusting the left rear seat. The scan tool showed B176D as an intermittent fault. Inspection revealed the pretensioner harness ran too tight beneath the seat rail. Moving the seat forward and rearward pinched the harness against the rail, damaging the insulation and partially breaking the internal copper strands, causing an intermittent open. Repair: Soldered the broken wiring and insulated with heat-shrink tubing, rerouted the harness, added corrugated tubing protection, and ensured clearance throughout the full seat travel. Fault resolved.
BYD DTC AI Analysis

Replaced pretensioner assembly due to internal open circuit

✓
Vehicle: BYD Tang DM 2021. After accident repairs, the airbag warning light remained on. DTC B176D-00. Checked connectors and wiring harness continuity—normal. Pretensioner terminal resistance measured infinite (open circuit). Disassembled pretensioner: internal squib pins fractured from impact. Pretensioners are single-use safety components; internal open circuit cannot be repaired. Replaced left rear seatbelt pretensioner assembly (with retractor). Cleared fault code; system normal. After replacement, perform pretensioner resistance calibration in 'Configuration and Settings' using a diagnostic tool.
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.