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 8

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 8 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 8
  • 1Worn insulation on the pretensioner wiring harness under the left middle-row seat or inside the B-pillar contacts a body power wire (e.g., constant 12V), causing a short circuit.
  • 2Wiring harness crushed during seat adjustment or removal, causing the internal copper wire to short to power.
  • 3Water ingress, oxidation, or deformed pins at 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 assembly; ignition 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 battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Locate the component: Remove the left middle-row seat or B-pillar lower trim panel, and find the seat belt pretensioner and its yellow dedicated connector (usually includes a shorting bar).
  • 3
    Visual inspection: Inspect the wiring harness sleeve for damage or burns. Check the connector for water ingress, corrosion, or backed-out pins. Specifically inspect the 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 pretensioner resistance; it must be 2.0-3.0Ω.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it using 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 ensure a waterproof seal.
  • 6
    Component replacement: If the pretensioner body has a short circuit, replace the entire seat belt assembly (including the pretensioner). Do not attempt repair.
  • 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 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.