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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) — Qin Plus

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.

3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Worn insulation on the pretensioner wiring harness under the left middle-row seat or inside the B-pillar contacts a vehicle power wire (such as constant 12V), causing a short circuit.
  • 2Seat adjustment or removal crushed the wiring harness, shorting the internal copper wire 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.
  • 4Seat belt pretensioner internal fault; igniter bridge wire shorted to metal housing or positive terminal.
  • 5Airbag Control Unit (ACU) internal drive circuit fault, falsely reporting a short circuit fault.
What To Do
  • 1
    Safety preparation: Disconnect the 12V 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 lower B-pillar trim. 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, and check the connector for water ingress, corrosion, or backed-out pins. Specifically check 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 should be no continuity. Measure the resistance of the pretensioner; it should 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 verify the waterproof seal.
  • 6
    Component replacement: If the pretensioner unit has a short circuit, replace the entire seat belt assembly (including the pretensioner). Do not 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 Qin Plus Fault Codes
B1108For 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.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.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.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.
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.