AU
B177A1B

DTC B177A1B indicates the SRS (Supplemental Restraint System/airbag system) control module detects an open circuit or resistance exceeding the normal threshold (typically >6Ω or infinite) in the left middle-row seat belt pretensioner circuit — Seal U

Safety System

DTC B177A1B indicates the SRS (Supplemental Restraint System/airbag system) control module detects an open circuit or resistance exceeding the normal threshold (typically >6Ω or infinite) in the left middle-row seat belt pretensioner circuit.

The '1B' sub-code in the BYD diagnostic system typically indicates excessive circuit resistance or an open circuit, causing the ECU to determine the pretensioner is 'not present' or has lost communication.

This fault prevents the system from triggering the left middle-row seat belt pretensioner pyrotechnic device during a collision, increasing occupant forward travel and reducing crash protection.

The system illuminates the instrument cluster airbag fault warning light (SRS light) and may disable the deployment logic for the corresponding side airbag or curtain airbag.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Left second-row seat belt pretensioner connector loose, terminal backed out, or poor contact (commonly caused by failing to reconnect after seat removal/repair or carpet cleaning, or by passenger kicks)
  • 2Pretensioner gas generator internal open circuit (internal igniter open circuit due to component aging, moisture ingress, or impact)
  • 3Wiring harness under the seat slide rail or B-pillar worn or broken (long-term fore/aft seat adjustment causes harness fatigue, or metal edges cut the insulation).
  • 4Water ingress or high humidity causes connector terminal corrosion and oxidation, resulting in high resistance or an open circuit.
  • 5SRS control module internal detection circuit fault (false alarm due to outdated software version, lost calibration data, or damaged hardware sampling circuit)
What To Do
  • 1
    Use the genuine BYD diagnostic tool (VDS2000/3000) to read all fault codes and freeze frame data, verify the environmental conditions when the fault occurred (temperature, vehicle speed, etc.), and distinguish between current and history faults.
  • 2
    Perform safety procedure: Disconnect the 12V battery negative terminal and wait at least 3 minutes (5 minutes for some models) to fully discharge the SRS system energy storage capacitor and prevent accidental airbag deployment.
  • 3
    Locate the left middle-row seat belt pretensioner (usually integrated into the seat belt retractor or below the B-pillar). Visually inspect the connector (usually marked in yellow) for looseness, backed-out pins, water ingress, or corrosion.
  • 4
    Use a multimeter to measure the resistance between the pretensioner terminals. The standard value is 1.8 Ω - 3.5 Ω. If the reading is infinity (OL) or >6 Ω, this confirms an open circuit in the pretensioner body or wiring.
  • 5
    Disconnect the pretensioner connector and measure continuity between the wiring harness side and the SRS ECU. Focus inspection on the seat slide rail mounting points, worn sections of the wiring harness corrugated conduit, and the body floor wiring harness clips.
  • 6
    If the pretensioner unit resistance is abnormal, replace the left middle-row seat belt assembly (the pretensioner is typically non-repairable, requiring replacement of the entire seat belt assembly). Verify the new part's resistance specification matches the original vehicle.
  • 7
    If the wiring harness has an open circuit, repair by soldering (do not simply twist wires together). Use dual-wall heat-shrink tubing for waterproof insulation and re-secure the wiring harness to prevent interference with moving parts.
  • 8
    Reconnect all connections and turn on the power. Use the diagnostic tool to clear the fault code and perform the 'SRS system self-check' or 'component test' to confirm B177A1B does not return.
  • 9
    Perform a road test to verify the repair: simulate moving the seat forward and backward, observe whether the fault light illuminates intermittently, and confirm complete fault resolution.
Real-World Cases
BYD DTC AI Analysis

BYD E2 left second-row pretensioner connector oxidised, causing open circuit

✓
The airbag warning light on the instrument panel remained illuminated. VDS showed DTC B177A1B (left middle row seatbelt pretensioner not present). Removed the left middle row seat and found the yellow pretensioner connector terminals severely oxidised due to previous water ingress; green copper corrosion caused infinite contact resistance. Cleaned the terminals with electronic contact cleaner, applied specialist conductive paste and waterproof silicone grease, then reconnected the connector. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Qin EV seat rail wear caused pretensioner harness open circuit

✓
Owner reported the airbag warning light illuminates intermittently on rough roads and clears on smooth surfaces. Retrieved DTC B177A1B as an intermittent fault. Inspection found the wiring harness protective sleeve damaged at the rear mounting point of the left second-row seat rail. When adjusting the seat fore-aft, the metal rail edge cut the harness, causing the internal copper wires to partially sever (intermittent continuity). Repaired the harness and rerouted the wiring, added rubber protective sleeves and securing clips to prevent contact with the rail. Fault fully resolved.
Original source ↗
BYD DTC AI Analysis

BYD E3 Airbag Control Module Spurious Fault

✓
No collision repair history. SRS warning light suddenly illuminated on the dash. Retrieved DTCs B177A1B (left middle row), B177B1B (right middle row), and other pretensioner faults. All pretensioner resistance measured 2.0Ω (normal); wiring harness continuity normal. Diagnosed as SRS ECU internal software bug causing false alarm. Performed SRS module software upgrade using VDS (V1.2 to V1.5), cleared DTCs. Fault has not recurred.
Original source ↗
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.