AU
B176C

DTC B176C indicates the left rear seat belt pretensioner circuit resistance exceeds the normal threshold set by the SRS ECU (typically >3 — Seal U

Safety System

DTC B176C indicates the left rear seat belt pretensioner circuit resistance exceeds the normal threshold set by the SRS ECU (typically >3.5-4.0Ω; normal value is 2.0-3.0Ω).

The pretensioner is a pyrotechnic safety device containing an igniter and a gas generator.

Excessive resistance indicates a high resistance or open circuit condition within the circuit.

Consequently, during a collision, the SRS ECU may fail to ignite the pretensioner charge.

This failure prevents the seat belt from tightening, increases occupant forward displacement, and severely reduces crash protection.

Additionally, the SRS system enters degraded mode, illuminates the airbag warning lamp continuously, and may disable the related side airbag function on certain models.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Left rear seat belt pretensioner internal igniter aging or open circuit: Long-term inactivity or moisture exposure oxidizes the internal bridge wire, causing resistance to increase gradually over time until it exceeds the threshold.
  • 2Poor connector contact: The pretensioner connector near the B-pillar or C-pillar (usually marked SR11 or P11) is loose, oxidized, corroded from water ingress, or has backed-out terminals, increasing contact resistance.
  • 3Wiring harness damage: Frequent door opening and closing partially breaks internal copper strands at the left rear door hinge, or B-pillar trim panel removal and installation pinches the harness, creating a high-resistance path.
  • 4Poor ground connection between the pretensioner and vehicle body: Loose or oxidized pretensioner housing ground point causes abnormal circuit resistance (for low-side drive pretensioner circuits).
  • 5SRS ECU internal sampling circuit fault: Faulty ECU internal A/D converter or sampling resistor falsely reports excessive resistance (rule out via cross-checking).
What To Do
  • 1
    Safety Preparation: Disconnect the negative battery terminal and wait at least 3 minutes to fully discharge the SRS system capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Connect the diagnostic tool and read the DTCs. Confirm B176C is a current fault (Active), not a history fault, and record the resistance value in the freeze frame data.
  • 3
    Visual inspection: Remove the left rear B-pillar lower trim panel. Inspect the pretensioner connector for looseness, water ingress, terminal corrosion, or backed-out pins. Clean and reconnect as necessary.
  • 4
    Pretensioner resistance measurement: Disconnect the connector and use a digital multimeter to directly measure the resistance across the two pretensioner terminals. Normal range: 2.0-3.0 Ω. If the resistance is >4 Ω or displays OL (open circuit), replace the left rear seat belt pretensioner assembly (usually integrated with the seat belt retractor).
  • 5
    Harness continuity test: Measure the harness resistance between the pretensioner connector and the SRS ECU (usually located under the center console or in the front compartment). Resistance must be <1.0 Ω. If resistance is too high, repair or replace the harness.
  • 6
    Insulation check: Measure the pretensioner wiring harness resistance to ground. Resistance must be >1 MΩ. If a short to ground exists, repair the damaged section of the wiring harness.
  • 7
    Cross-check: If the above checks are normal, test with a known-good pretensioner. If the fault code clears, the pretensioner has an internal fault. Otherwise, check the SRS ECU.
  • 8
    System reset: Install all components, connect the battery, and clear the fault codes. Turn the ignition switch to the ON position and verify the SRS warning lamp turns off after 6 seconds. Perform a road test to confirm the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

B-pillar trim removed and refitted with connector not locked, causing high resistance

✓
A 2020 BYD Song MAX triggered DTC B176C following left rear door respray. The white 4-pin pretensioner connector wasn't fully locked after the B-pillar trim was removed and refitted—just loose. Reseated the plug until it clicked; resistance returned to 2.3Ω. Cleared the code; SRS light went out. Common after accident repairs since the connector hides behind the trim and is easy to miss.
BYD DTC AI Analysis

Electrical connectors oxidised and corroded after water submersion

✓
A 2019 Yuan EV's airbag warning light came on one day after driving through heavy floodwater. Scan showed DTC B176C. Removed the left rear floor trim and found water in the seatbelt pretensioner harness connector; the copper pins showed green oxidation. Cleaned the connector with electrical contact cleaner, applied conductive grease, and reinstalled it. The fault cleared temporarily but returned a week later. Replaced the pretensioner harness connector and waterproofed it—problem solved. Inspect SRS system connector sealing immediately on any flood-damaged vehicle.
BYD DTC AI Analysis

Pretensioner internal igniter open circuit due to aging

✓
A 2019 BYD Qin gasoline variant (commercial fleet vehicle, 280,000 km) logged active fault code B176C. Resistance at the pretensioner connector read OL (open circuit). Disassembly revealed a fractured igniter bridge wire. Replaced the left rear seatbelt assembly (including pretensioner); fault cleared. Root cause: Metal fatigue in the igniter from prolonged intensive use and wide cabin temperature swings. High-mileage vehicles require regular SRS component inspections.
BYD DTC AI Analysis

Wiring harness internal break at left rear door hinge

✓
A 2021 BYD Tang DM exhibited intermittent SRS warning light illumination when opening or closing the left rear door. The scan tool displayed B176C intermittent fault. Static resistance measured normal, but resistance fluctuated when repeatedly cycling the door. Stripping back the wiring harness protective sleeve at the left front door hinge revealed two copper wires partially fractured, with only a few thin strands remaining connected. Replaced the wiring harness section from body to door (including pre-tensioner circuit). Repeated door cycling confirmed stable resistance and resolved the fault. This is a common failure point on BYD models; frequent door operation causes wiring harness fatigue.
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.