AU
B177B

DTC B177B indicates the control circuit for the second-row left seatbelt pretensioner has shorted to the vehicle power supply positive (B+) — Atto 3

Safety System

DTC B177B indicates the control circuit for the second-row left seatbelt pretensioner has shorted to the vehicle power supply positive (B+).

In the BYD SRS system, the pretensioner operates as a pyrotechnic actuator.

Under normal conditions, the SRS ECU controls the pretensioner via a low-side drive and, during a collision, supplies an instant high current (approximately 2-3A) to ignite the gas generator.

A short to power means the circuit continuously carries an abnormally high potential (12V).

This may result in the following: 1) The SRS ECU falsely detects a permanent trigger state or wiring fault, activating the system protection mechanism. 2) The SRS ECU disables the entire airbag system, preventing protection during a collision. 3) The pretensioner accidentally deploys or the ECU internal driver circuit burns out in extreme cases.

This is a hard fault and requires immediate repair.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Mechanical wear of the under-seat wiring harness: During forward and backward adjustment of the left middle-row seat (common on 7-seat models such as the Song MAX and Tang), the pretensioner wiring harness rubs against the seat slide rail or metal bracket. Prolonged friction damages the insulation and causes a short circuit to the seat power wire (constant live).
  • 2Connector water ingress and corrosion: Water enters the pretensioner connector (usually a yellow waterproof plug) located under the seat or on the B-pillar due to vehicle wading, sunroof leaks, or deep interior cleaning, creating an electrolytic conductive path between the power and signal terminals.
  • 3Pretensioner internal short circuit: Insulation breakdown between the gas generator internal igniter bridge wire and the metal housing, or moisture in the igniter charge holder causing ionic conduction (common in older vehicles or high-temperature, high-humidity environments).
  • 4Modification or repair damage: During seat heating/ventilation retrofits, carpet replacement, or accident repairs, fixing screws pierce the wiring harness, or the seat frame pinches the wiring harness against the vehicle body, damaging the insulation.
  • 5Internal SRS ECU fault: Power MOSFET breakdown or freewheeling diode short circuit in the pretensioner drive circuit causes a continuous high-level output (less common, usually occurs with multiple pretensioner fault codes).
What To Do
  • 1
    Safety preparation: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS system energy storage capacitor and prevent accidental deployment.
  • 2
    Fault Confirmation: Use a BYD VDS or Launch X-431 diagnostic tool to read the fault code. Confirm B177B is a Current DTC, not a History DTC. Record freeze frame data to observe vehicle status at the time of the fault.
  • 3
    Visual inspection: Remove the left middle-row seat (usually requires removing 4 retaining bolts). Inspect the pretensioner wiring harness (wrapped in yellow corrugated conduit) under the seat, below the B-pillar, and inside the floor wiring channel for abrasion, cuts, burn damage, or water ingress. Focus on interference points between the wiring harness and the seat slide rail.
  • 4
    Pretensioner unit inspection: Disconnect the pretensioner connector (usually under the seat). Use a multimeter to measure the resistance between the two terminals of the pretensioner unit. Standard value: 2.0-3.0Ω (at 20℃). If the resistance is 0Ω or infinite, replace the pretensioner assembly. (Note: Do not use a multimeter resistance setting to measure a wiring harness connected to the ECU.)
  • 5
    Harness short to power check: Turn the ignition switch to ON (do not start the vehicle). Measure the voltage between the two terminals of the pretensioner harness-side (ECU side) connector and body ground. The voltage should be 0 V or close to 0 V (<0.1 V). A reading of 12 V battery voltage indicates the harness is shorted to power.
  • 6
    Continuity test: Disconnect the SRS ECU connector (located under the center console or inside the armrest box). Use a multimeter to measure continuity between the pretensioner wiring harness (ECU end to pretensioner end) and a known power wire (such as the seat heater power supply or a constant power fuse). If continuity exists, locate the short circuit.
  • 7
    Insulation resistance test: Measure the insulation resistance from the two pretensioner harness wires to body ground. Resistance must be greater than 1 MΩ. If insulation is poor, strip back the corrugated conduit section by section to locate the damaged point.
  • 8
    Replacement verification: Replace the original pretensioner with a dedicated 2Ω resistor (or a known-good pretensioner). Clear the fault code, then cycle the ignition switch 3 times. If the fault disappears, this confirms an internal short circuit in the pretensioner. Replace with a genuine pretensioner (replace the seat belt assembly simultaneously, as the pretensioner usually integrates with the seat belt retractor).
  • 9
    Wiring harness repair: If the harness is damaged, cut out the damaged section and repair it using heat-shrink tubing or high-temperature insulating tape (the pretensioner circuit carries high current; ordinary PVC tape melts easily). When rerouting the harness, maintain a clearance of at least 20mm from the seat slide rail and secure it with cable ties.
  • 10
    System verification: Reconnect all connectors and the battery. Turn the ignition switch ON and confirm the instrument cluster SRS warning light turns off after 6 seconds. Use the diagnostic tool to perform the 'Pretensioner Function Test' (Trigger Test) to verify ECU drive capability, then perform a road test for final verification.
Real-World Cases
BYD DTC AI Analysis

Worn seat adjustment rails chafed the wiring harness on the Song MAX, causing a short circuit.

✓
A 2019 Song MAX DM, 42,000 km, SRS warning light constantly on. Read DTC B177B (active fault). Found the left middle seat pre-tensioner harness (yellow) chafing against the seat slide rail bracket underneath. The insulation had worn through, exposing copper that shorted to the seat heater power wire (red, constant live). Cause: Long-term full-load use with frequent fore-aft seat adjustment shifted the harness. Repair: Cut out the worn section and re-crimped terminals. Rerouted the harness outside the slide rail, added a rubber sleeve and secured with cable ties to maintain 30 mm clearance from moving parts. Replaced the pre-tensioner assembly (potential internal damage from the short circuit).
BYD DTC AI Analysis

Tang EV connectors corroded after water ingress from wading

✓
2021 Tang EV. After driving through standing water during heavy rain, the instrument cluster displayed "Check Airbag System" with DTC B177B. Teardown revealed visible water inside the pretensioner connector (yellow 2-pin plug) beneath the left middle seat; terminals were oxidised and blackened. Cause: Wading depth exceeded floor height. Water entered the connector through the wiring harness corrugated tube; electrolytes created a conductive path between the power and signal pins. Fix: Cleaned the terminals thoroughly with electronic contact cleaner, blew them dry with compressed air (do not use a heat gun—risk of plastic deformation), and applied conductive anti-corrosion grease. Checked the harness seal integrity and replaced the connector assembly if necessary. Cleared the DTC and monitored for one week with no recurrence.
BYD DTC AI Analysis

Yuan EV short circuit caused by bent pins after accident repair

✓
2019 BYD Yuan EV. Rear-ended on the left rear side and repaired at an external workshop. After the shop replaced the left rear seatbelt assembly (including pretensioner), the instrument cluster lit the SRS warning light and set DTC B177B. Found bent pins on the new pretensioner harness connector from the repair work. The power supply pin (normally fed by the ECU, but contacting body power during connector handling) shorted to an adjacent constant-power wire (such as the seatbelt warning indicator supply). Fix: Straightened the bent pins, confirmed no fractures at the pin roots, and used a dedicated terminal removal tool to adjust the terminal positions. Verified normal pretensioner resistance and harness insulation, then reconnected the harness. After accident repairs, always perform a complete SRS system diagnosis rather than simply clearing fault codes.
BYD DTC AI Analysis

Qin Pro: Aftermarket aviation seat install damaged wiring harness

✓
2020 BYD Qin Pro (petrol). Owner installed an aftermarket aviation-style seat with leg rest and heating in the left mid-row position. DTC B177B appeared the day after installation. Inspection found self-tapping screws for the seat mounting bracket had penetrated the floor wiring harness, piercing the pre-tensioner supply wire (black/yellow) and seat heating supply wire (red). Cause: No wiring harness probe used; blind drilling. Fix: Removed the aftermarket seat, replaced the damaged pre-tensioner wiring harness assembly (repair not recommended—multiple conductors damaged), and relocated seat mounting points to avoid the harness routing. Owner warned: SRS system wiring harnesses must never be modified or spliced.
BYD DTC AI Analysis

Intermittent short circuit causing sporadic warning light illumination

✓
A 2019 Song MAX came in with an intermittent SRS warning light that worsened on rough roads. The scanner showed B177B as an intermittent fault. Static harness tests checked out normal, but shaking the left middle-row seat reproduced the fault. Close inspection found the pretensioner harness under the B-pillar was cut by sharp burrs where it passes through the body sheet metal hole. Vehicle vibration caused the damaged insulation to touch body power lines intermittently. Repair: Rebuilt the insulation with double-layer heat shrink tubing. Installed a rubber grommet on the sheet metal hole and eliminated the burrs. Adjusted the harness clip positions to keep the wiring slack throughout the full range of seat movement.
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.