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 8

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 8
  • 1Loose pretensioner connector or poor contact: 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 below the B-pillar. On MPV models like the Song MAX, the large range of movement of the middle-row seats exposes the connector to mechanical stress.
  • 2Wiring harness broken or worn: Repeated bending of the pretensioner wiring 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. This commonly occurs on 7-seat versions of SUV models such as the Tang and Song.
  • 3Pretensioner assembly internal fault: The igniter or bridge wire inside the pretensioner has an open circuit. Causes include manufacturing defects, prolonged vibration, or failure to replace the unit after deployment in a previous accident.
  • 4SRS control unit false report: An internal fault in the airbag ECU detection circuit or a software calibration issue causes the system to incorrectly identify the pretensioner as missing. This occurs mostly in specific software versions of early Yuan EV or Qin models.
  • 5Incorrect wiring after accident repair: Following a collision or interior trim removal and installation, the technician failed to correctly connect the left second-row pretensioner connector or used a mismatched resistor substitute to bypass the fault, causing the system to detect an abnormality.
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 BYD dedicated diagnostic tool (VDS2000 or Launch X431) to read all fault codes. Confirm whether B177A is an Active or History fault, and record the freeze frame data to review the vehicle status at the time of the fault.
  • 2
    Visual inspection and connector check: Inspect the left middle-row seat belt assembly for signs of impact damage or disassembly. Locate the pretensioner connector (usually on the lower inner side of the seat or inside the B-pillar trim). Verify the connector is fully locked and inspect the terminals for oxidation, backed-out pins, or corrosion. If necessary, clean with electrical contact cleaner and reconnect.
  • 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. Standard value: 1.5-2.5Ω. 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 is severely worn, replace the entire pretensioner wiring harness assembly to prevent recurrence.
  • 5
    Component replacement and system calibration: If the pretensioner unit is faulty, replace the left middle-row seat belt assembly (including the pretensioner). After installing the new part, use the diagnostic tool to execute 'SRS system configuration' or 'ECU coding' (some models require writing the vehicle VIN and configuration code). Next, clear the fault code and perform a system self-check (usually requiring 3 ignition switch cycles). Finally, perform a vehicle collision simulation test (use a dedicated dummy resistor 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 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.