AU
B177B12

DTC B177B12 indicates a short circuit to vehicle power (+12V B+) in the firing circuit of the left second-row seat belt pretensioner within the Supplemental Restraint System (SRS) — Atto 3

Safety System

DTC B177B12 indicates a short circuit to vehicle power (+12V B+) in the firing circuit of the left second-row seat belt pretensioner within the Supplemental Restraint System (SRS).

The Airbag Control Unit (ACU) controls the pretensioner, a pyrotechnic device with a normal resistance of approximately 2.0-3.0Ω.

The ACU determines a short to power when it detects abnormally low circuit resistance (typically <1.0Ω) or abnormal supply voltage.

This severe safety fault prevents the pretensioner from firing correctly during a collision, or causes the system to lock it out due to inadvertent deployment risks.

Root causes include damaged wiring harness insulation contacting a power wire, misaligned connector terminals, or a short circuit between the pretensioner internal bridge wire and the housing.

The B177B12 fault code on the Atto 3 indicates a high-severity SRS fault: a short to 12V power in the left second-row seat belt pretensioner firing circuit. This can stop the pretensioner working correctly or cause the system to disable it for safety. Common causes include damaged or crushed wiring under the seat or in the B-pillar. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Worn insulation on the pretensioner wiring harness under the left middle-row seat or inside the B-pillar contacts a body power wire (such as constant 12V), causing a short circuit.
  • 2Seat adjustment or removal crushed the wiring harness, causing the internal copper wire to short to power.
  • 3Water ingress, oxidation, or deformed pins in the pretensioner connector (usually located under the seat or inside the B-pillar trim), causing a short circuit between terminals.
  • 4Internal fault in the seat belt pretensioner unit: igniter bridge wire shorted to metal housing or positive terminal.
  • 5Airbag Control Unit (ACU) internal drive circuit fault, falsely reporting a short circuit.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to completely discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Locate the component: Remove the left middle-row seat or B-pillar lower trim, then find the seat belt pretensioner and its yellow dedicated connector (usually contains a shorting bar).
  • 3
    Visual inspection: Check the wiring harness sleeve for damage or burns, and the connector for water ingress, corrosion, or backed-out pins. Pay special attention to wiring harness wear points at the seat slide rail.
  • 4
    Disconnect and measure: Disconnect the ACU from the pretensioner. Use a multimeter to measure continuity between the pretensioner-side harness and power (B+); there must be no continuity. Measure the resistance of the pretensioner body; the value must be 2.0-3.0Ω.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it with high-temperature tape (SRS-specific) or replace the wiring harness. Maintain a safe distance from the power cable. If the connector is damaged, replace the connector and verify it is waterproof.
  • 6
    Component replacement: If the pretensioner body has a short circuit, replace the entire seat belt assembly (including the pretensioner). Do not attempt repairs.
  • 7
    System reset: Reconnect all components and the battery. Use VDS or a dedicated diagnostic tool to clear the fault code. Perform an SRS system self-check and verify B177B12 does not return.
Real-World Cases
BYD DTC AI Analysis

E2 Water accumulation under left rear seat caused pretensioner short circuit

✓
A 2019 BYD E2 had an SRS warning light stay on. Retrieved DTC B177B12. Found water stains beneath the left rear seat, likely from a rear passenger spilling liquid. Removed the seat and found electrolytic corrosion inside the pretensioner connector (yellow). Pins 1 (positive supply) and 2 (pretensioner positive) were shorted. Cleaned and dried the wiring harness, installed a new waterproof connector, and measured normal resistance. Cleared the DTC and the system returned to normal.
BYD DTC AI Analysis

Qin EV aftermarket carpet chafed through wiring harness, causing short circuit

✓
2019 Qin EV. Owner reported the SRS warning lamp lit intermittently; DTC B177B12 intermittent. Found aftermarket full-surround floor mats installed; mounting screws pierced the wiring harness sheath beneath the left middle seat, chafing the pretensioner supply wire and shorting it to constant power. Replaced the entire SRS wiring harness, rerouted wiring to specification and added protective sheathing. Fault resolved.
BYD DTC AI Analysis

Replaced pretensioner assembly due to internal short circuit (E3)

✓
2020 BYD E3. B177B12 appeared after accident repairs. Found a short to power at the left second-row pretensioner harness. Disconnected the pretensioner and measured 0.5Ω resistance with continuity to the housing. The internal bridge wire had shorted to the housing. Replaced the left second-row seat belt assembly (including pretensioner), performed coding and matching, and cleared the fault. The pretensioner fired during the accident but was not replaced; internal damage caused the short.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.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.
Common problems reported for Atto 3 →Atto 3 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.