AU
B177A

DTC B177A indicates the SRS (Supplemental Restraint System) control unit fails to detect the left middle-row (typically left second-row) seat belt pretensioner during self-diagnosis — Atto 3

Safety System

DTC B177A indicates the SRS (Supplemental Restraint System) control unit fails to detect the left middle-row (typically left second-row) seat belt pretensioner during self-diagnosis.

This is an open-circuit fault; the ECU detects infinite pretensioner circuit resistance or resistance outside the normal threshold (standard value: 1.5-2.5Ω).

The pretensioner is a key actuator in the passive safety system.

During a frontal collision, a pyrotechnic igniter instantly tightens the seat belt to eliminate slack between the occupant and the belt.

This fault indicates an open pretensioner circuit, preventing pretensioner deployment for this seat during a collision.

This condition may also disrupt the coordinated deployment logic of the same-side or related airbags. "Not present" in this fault code does not indicate a missing physical component; it designates an electrical "communication loss" or "open circuit".

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Loose or poor contact at pretensioner connector: Frequent forward and backward sliding or folding of the left middle-row seat can loosen the pretensioner wiring harness connector located under the seat or lower B-pillar. The large movement range of middle-row seats on MPV models like the Song MAX subjects the connector to mechanical stress.
  • 2Harness breakage or wear: Repeated bending of the pretensioner harness near the seat rails causes fatigue fractures in the internal copper conductors, or the metal seat frame edge wears through the harness insulation, causing an open circuit. Common on 7-seat versions of SUV models such as Tang and Song.
  • 3Internal pretensioner fault: The igniter or bridge wire inside the pretensioner has an open circuit. Possible causes include manufacturing defects, prolonged vibration, or failure to replace the unit after a previous deployment.
  • 4SRS control unit false report: An airbag ECU internal detection circuit fault or software calibration issue causes the ECU to incorrectly identify the pretensioner as missing. Common in specific software versions of early Yuan EV or Qin models.
  • 5Incorrect wiring after accident repair: Following a collision or interior disassembly and reassembly, the technician failed to properly connect the left second-row pretensioner connector, or used a mismatched substitute resistor to bypass the fault, causing abnormal system detection.
What To Do
  • 1
    Safety Preparation and Initial Diagnosis: Disconnect the negative battery terminal and wait at least 3 minutes to fully discharge the SRS capacitor. Use a dedicated BYD diagnostic tool (VDS2000 or Launch X431) to read all fault codes. Confirm whether B177A is a current fault (Active) or a history fault (History). Record the freeze frame data to view the vehicle status at the time of the fault.
  • 2
    Visual and connector inspection: Inspect the left middle-row seat belt assembly for signs of impact or disassembly. Locate the pretensioner connector (usually on the lower inner side of the seat or inside the B-pillar trim). Verify the connector locks fully. Inspect the terminals for oxidation, backed-out pins, or corrosion. Clean with electrical contact cleaner and reconnect if necessary.
  • 3
    Circuit measurement and resistance test: Disconnect the pretensioner connector. Use a multimeter to measure the resistance across the terminals on the pretensioner-side connector. The standard value is 1.5-2.5 Ω. If the reading is infinite (OL), replace the pretensioner assembly. If the resistance is normal, measure continuity between the harness-side connector and the SRS control unit to check for an open circuit or short to ground.
  • 4
    Wiring harness repair and insulation: If the wiring harness is broken at any point (especially at the seat slide rail bend), strip the harness sheath, solder the break, and apply double-layer heat-shrink tubing or waterproof tape for insulation. If the wiring harness shows severe wear, replace the entire pretensioner wiring harness assembly to prevent recurrence.
  • 5
    Component replacement and system calibration: If testing confirms the pretensioner is faulty, replace the left middle-row seat belt assembly (including the pretensioner). After installing the new part, use a diagnostic tool to execute 'SRS system configuration' or 'ECU coding' (some models require writing the vehicle VIN and configuration code). Then clear the fault codes and perform a system self-check (usually requires cycling the ignition switch 3 times). Finally, perform a vehicle crash simulation test (use a dedicated resistor substitute to verify normal system communication).
Real-World Cases
BYD DTC AI Analysis

Song MAX left second-row seat rail worn through pretensioner wiring harness

✓
A 2019 BYD Song MAX DM with 42,000 km had the airbag warning light illuminated. Scanning revealed code B177A. Removing the left middle seat revealed the pre-tensioner wiring harness had chafed against the metal edge at the front of the seat rail, wearing through the insulation and completely severing all three copper cores. Replaced the pre-tensioner wiring harness assembly (part number: SA-585XXXX), added a rubber protective sleeve and cable ties at the contact point, and rerouted the wiring to prevent future abrasion. Cleared the fault codes and the system returned to normal.
BYD DTC AI Analysis

Tang DM pretensioner connector not connected after accident repair

✓
2021 BYD Tang DM. Following right-front collision repairs at a non-authorised workshop, the instrument cluster displayed a constant airbag warning light. Diagnosis revealed DTC B177A and a right-front impact sensor fault. Inspection found the left second-row seat had been removed for painting during the accident repair. Upon reinstallation, the technician had not fully seated the pretensioner connector, leaving it merely hanging on the clip and causing a loose connection. Reconnected the connector and confirmed the locking tab engaged. Performed 'Crash History Clear' and 'SRS System Reset' using a diagnostic tool. Fault resolved.
BYD DTC AI Analysis

Aftermarket second-row seat modification on Yuan EV dislodged the connector

✓
2019 BYD Yuan EV535. Owner self-installed a heating pad on the left middle row seat but didn't properly refit the trim panels afterward. Seat adjustment pulled the wiring, causing the pretensioner connector to come completely loose. Airbag warning light came on intermittently, particularly on rough roads. Found the connector partially detached with slight terminal oxidation. Fix: Cleaned the terminals, applied conductive grease, re-secured the connector and used cable ties to take up harness slack so seat movement wouldn't pull on the wires.
BYD DTC AI Analysis

BYD Song MAX Pretensioner Internal Open Circuit Replacement Case

✓
A 2019 Song MAX petrol with no accident history. The airbag warning light illuminated suddenly. Resistance measured infinite at the left second-row pre-tensioner connector. Disconnected the pre-tensioner; the body alone still measured infinite resistance, confirming the internal bridge wire had fractured. Replaced the genuine seatbelt assembly (including pre-tensioner, part number: SA-582XXXX). Fault cleared. Analysis suggests internal propellant moisture or long-term micro-vibration caused bridge wire fatigue. Sporadic component failure.
BYD DTC AI Analysis

Qin Pro SRS Control Unit Software Upgrade for False Alarm

✓
2020 Qin Pro developed intermittent B177A faults. Checked wiring harness and pretensioner resistance – both normal. Connectors normal. Consulted BYD TSB: confirmed early-batch SRS control unit (ACU) software calibration issue causes false middle-row pretensioner missing detection during low-temperature cold starts. Action: Upgraded ACU software to V3.2.1 or higher using VDS. No further faults after update.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.