AU
B176C

DTC B176C indicates the left rear seat belt pretensioner circuit resistance exceeds the normal threshold set by the SRS ECU (typically >3 — Atto 8

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.

The B176C fault code on the Atto 8 indicates high resistance in the left rear seat belt pretensioner circuit, which can stop the pretensioner from tightening the belt in a crash. Severity is medium, but the airbag warning light may stay on and SRS protection can be reduced. Common causes include an aging/open pretensioner igniter or poor connector contact from loose pins, corrosion, or moisture. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Left rear seat belt pretensioner internal igniter aging or open circuit: Prolonged inactivity or moisture exposure oxidizes the internal bridge wire, gradually increasing resistance 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, has backed-out pins, oxidation, or water ingress corrosion, causing increased contact resistance.
  • 3Wiring harness damage: Frequent opening and closing of the left rear door partially breaks the internal copper strands of the wiring harness at the hinge, or B-pillar trim panel removal and installation pinches the harness, creating a high-resistance path.
  • 4Poor ground connection between the pretensioner and the body: A loose or oxidized pretensioner housing ground point causes abnormal circuit resistance (for low-side drive pretensioner circuits).
  • 5SRS ECU internal sampling circuit fault: Abnormal internal ECU A/D converter or sampling resistor falsely reporting 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 allow the SRS system capacitor to fully discharge and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Connect the diagnostic tool to read the DTC. Confirm B176C is a current (Active) fault, not a history fault. 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, corroded terminals, or backed-out pins. Clean and reconnect as necessary.
  • 4
    Pretensioner resistance measurement: Disconnect the connector. Use a digital multimeter to measure resistance directly across the two pretensioner terminals. Normal range: 2.0-3.0 Ω. If 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 harness resistance between the pretensioner connector and the SRS ECU (usually under the centre 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-to-ground resistance. Resistance must be >1MΩ. If a short to ground exists, repair the damaged wiring harness section.
  • 7
    Cross-check: If all above checks are normal, install a known-good pretensioner and test. 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 code. 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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.