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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) — Seal 6 EV

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 Seal 6 EV indicates a high-severity SRS fault: a short to 12V power in the left second-row seat belt pretensioner firing circuit. Common causes include damaged or crushed wiring under the seat or in the B-pillar, or connector/terminal damage causing contact with a power wire. The recommended fix is to visit an authorised dealer for safe diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 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.
  • 2Wiring harness crushed during seat adjustment or removal, causing 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.
  • 4Seat belt pretensioner unit internal fault; igniter bridge wire short to metal housing or positive terminal.
  • 5Airbag Control Unit (ACU) internal drive circuit fault reporting a false 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 lower B-pillar trim panel. Find the seat belt pretensioner and its dedicated yellow 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. Specifically check for 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 wiring harness and the power supply (B+); there must be no continuity. Measure the resistance of the pretensioner itself; the resistance must be between 2.0 and 3.0Ω.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it with high-temperature tape (SRS-specific) or replace the harness. Route the harness to maintain a safe distance from the power cable. If the connector is damaged, replace the connector and ensure waterproofing.
  • 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 confirm 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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.