AU
B177C11

DTC B177C11 indicates a short circuit to body ground (GND) in the ignition trigger circuit of the left middle-row (second-row left) seat belt pretensioner — Atto 3

Safety System

DTC B177C11 indicates a short circuit to body ground (GND) in the ignition trigger circuit of the left middle-row (second-row left) seat belt pretensioner.

The pretensioner is a critical component of the SRS (Supplemental Restraint System).

It contains a pyrotechnic ignition device (squib).

During a collision, the airbag control unit (ACU) triggers the squib with a specific current to instantly tighten the seat belt and protect the occupant.

This DTC indicates the ACU detected abnormally low resistance in the pretensioner circuit (close to 0Ω), falling outside the normal threshold (typically 1.5–3.0Ω).

Potential causes include damaged wiring insulation contacting the vehicle frame, water ingress at the connector shorting the terminal to ground, or an internal short between the pretensioner resistance wire and the metal housing.

This critical safety fault disables the pretensioner for the affected seat and may trigger the SRS fail-safe mode, preventing proper airbag deployment.

Extreme cases pose a risk of unintended heat generation or ignition.

The B177C11 fault code on the Atto 3 indicates a high-severity short to body ground in the second-row left seat belt pretensioner ignition circuit. This SRS fault can disable that pretensioner and may affect airbag system operation. Common causes include chafed or pinched wiring under the seat or B-pillar trim, or water/corrosion in the yellow pretensioner connector. 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 3
  • 1Frequent fore-and-aft seat adjustment or pinching chafes the wiring harness under the seat or inside the B-pillar trim panel, causing the exposed wire to contact the metal body frame and create a short to ground.
  • 2After vehicle wading, heavy rain, or deep interior cleaning, water enters the pretensioner connector (usually located under the seat, yellow plug), causing a short circuit or corrosion between the terminals or from terminal to ground.
  • 3Manufacturing defect or aging of the pyrotechnic squib inside the seat belt pretensioner, causing the resistance wire to short to the metal housing
  • 4Airbag control unit (SRS ECU) internal drive circuit or monitoring circuit fault causes a false detection, triggering a short-to-ground fault code.
  • 5During seat modification, floor mat installation, or third-party repairs, overly tight screws or cable ties damage the wiring harness insulation, or improper harness routing causes dynamic interference with the seat slide rails or metal brackets.
What To Do
  • 1
    Safety preparation: Switch the vehicle OFF, disconnect the battery negative terminal, and wait at least 90 seconds to ensure the SRS capacitor fully discharges. This prevents accidental pretensioner activation and personal injury during maintenance.
  • 2
    Visual inspection: Remove the left middle-row seat (or move the seat to the fully forward or rearmost position). Locate the seat belt pretensioner connector (usually a yellow waterproof plug) and inspect it for damage, water stains, burn marks, or terminal back-out.
  • 3
    Wiring inspection: Disconnect the pretensioner connector. Use a multimeter to measure the resistance between the wiring harness side (Vehicle Side) signal pin and body ground. The normal reading is infinite (OL). If the reading shows continuity (0Ω or close to 0Ω), inspect the wiring harness along its route for chafing points. Focus on the seat slide rail, inside the B-pillar trim panel, and the floor wiring harness channel.
  • 4
    Component testing: Measure the resistance between the two pins on the pretensioner side (Component Side). Normal resistance is 1.5-3.0 Ω. If the resistance is 0 Ω or extremely low (<1 Ω), the pretensioner has an internal short circuit. Replace the seat belt assembly (pretensioners are usually not available separately).
  • 5
    Repair verification: After repairing the wiring harness (cut out the damaged section, re-solder, insulate with heat-shrink tubing, and adjust routing to prevent interference) or replacing the pretensioner, reconnect the connector and reconnect the battery. Use the VDS or X431 diagnostic tool to clear the fault code and perform an SRS system self-check. Confirm DTC B177C11 does not return and the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

BYD E2 seat rail chafed through wiring harness causing short circuit

✓
A 2019 BYD E2 with 32,000 km showed a steady airbag warning light on the instrument panel. VDS scan retrieved DTC B177C11. Technicians removed the left rear seat and found the pretensioner wiring harness had chafed through at the left seat rail mounting bracket. Exposed copper wire contacted the metal frame, causing a short to ground. They cut away the damaged harness section, resoldered the joints, insulated with double-layer heat shrink tubing, and repositioned the harness retaining clips to clear the seat rail travel. Fault cleared.
BYD DTC AI Analysis

Qin EV pretensioner connector corroded after water ingress

✓
A 2019 Qin EV showed an airbag system warning on the dash at startup the day after driving through flooded roads in heavy rain. DTC B177C11. Found water and green corrosion inside the pretensioner connector under the left rear seat, causing a signal terminal short to ground. Cleaned the connector contacts with electronic cleaner, dried with compressed air, and applied conductive protectant. Measured normal pretensioner resistance, reconnected the wiring, cleared the fault code. Back to normal.
BYD DTC AI Analysis

Abnormal internal resistance in E3 model pretensioner

✓
A 2020 BYD E3 with no collision or water damage history had an intermittent DTC B177C11 that would not clear. Disconnected the pretensioner connector and measured the unit resistance: 0.3Ω cold (abnormally low). Tapping the seat belt retractor caused the resistance to fluctuate, confirming an internal igniter short. Replaced the left middle row seat belt assembly (including pretensioner) and ran SRS system configuration and calibration with a scan tool. Fault cleared permanently.
BYD DTC AI Analysis

Seat modification pinched wiring harness, causing short circuit

✓
A BYD E2 came in with the airbag warning light on after the owner fitted racing seats, code B177C11 stored. Mounting bolts crushed the seatbelt pretensioner harness during the modification, damaging the insulation and shorting it to ground. Repaired the harness, rerouted the wiring to maintain safe clearance from the seat frame. Fault resolved.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.