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 — Qin Plus

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.

The B177A1B fault code on the Qin Plus indicates a medium-severity problem in the SRS airbag system: the left middle-row seat belt pretensioner circuit is open or has excessive resistance. Common causes include a loose or poorly connected pretensioner connector, backed-out terminals, or an internal pretensioner fault. The recommended fix is to visit an authorised dealer for inspection, repair, and clearing the code. Typical repair cost data is available from recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Left middle-row seat belt pretensioner connector loose, terminal backed out, or poor contact (commonly caused by failing to reconnect the connector after seat removal for 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  • 5SRS control module internal detection circuit fault (outdated software version, lost calibration data, or damaged hardware sampling circuit causing false alarm)
What To Do
  • 1
    Use a genuine BYD diagnostic tool (VDS2000/3000) to read all fault codes and freeze frame data, verify the environmental conditions at the time of the fault (temperature, vehicle speed, etc.), and distinguish between current and history faults.
  • 2
    Perform safety procedure: Disconnect the 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 oxidation.
  • 4
    Measure the resistance between the pretensioner terminals using a multimeter: the standard value is 1.8Ω - 3.5Ω. If the reading shows infinity (OL) or >6Ω, this confirms an open circuit in the pretensioner body or wiring.
  • 5
    Disconnect the pretensioner connector and measure continuity between the harness side and the SRS ECU. Specifically inspect the seat slide rail mounting points, wear points on the corrugated harness conduit, and body floor harness clips.
  • 6
    If the pretensioner body resistance is abnormal, replace the left middle-row seat belt assembly (the pretensioner is typically non-serviceable; replace the entire seat belt assembly). Verify the new part matches the original vehicle's resistance specification.
  • 7
    If the wiring harness has an open circuit, repair it by soldering (never simply twist the wires together). Use dual-wall heat-shrink tubing for waterproof insulation and re-secure the harness to avoid interference with moving parts.
  • 8
    Reconnect all connections and turn on the power. Use the diagnostic tool to clear the fault code, perform the 'SRS system self-check' or 'component test', and verify B177A1B does not return.
  • 9
    Perform a road test to verify: Move the seat forward and backward, observe if the fault light illuminates intermittently, and confirm the fault is completely resolved.
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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.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.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.