AU
B179B

DTC B179B indicates a short-to-power fault in the front passenger seat belt pretensioner second-stage deployment circuit — Seal 6 EV

Safety System

DTC B179B indicates a short-to-power fault in the front passenger seat belt pretensioner second-stage deployment circuit.

In the BYD SRS (Supplemental Restraint System) architecture, the pretensioner utilizes a dual-stage ignition design: the first stage lightly retracts the seat belt during the initial collision phase, and the second stage forcefully retracts it to secure the occupant as the collision intensifies.

This DTC sets when the SRS control module detects an abnormally low-resistance path (typically <1Ω) between the front passenger pretensioner second-stage ignition circuit (usually marked as terminals P+ and P-) and vehicle power (B+) or ground (GND), causing the control module to identify a short circuit.

This fault results in the following: 1) the front passenger airbag system enters fail-safe mode, and the instrument cluster airbag warning lamp remains illuminated; 2) the second-stage pretensioner may fail to deploy or deploy unintentionally during an actual collision; 3) because the pretensioner is a pyrotechnic device (containing explosive propellant), continuous short-circuit current creates an accidental ignition and deflagration risk.

Therefore, the system classifies the fault as severe (Level 3) and prohibits vehicle operation.

The B179B fault code on the Seal 6 EV indicates a high-severity short-to-power fault in the front passenger seat belt pretensioner second-stage deployment circuit. This can disable proper SRS operation and may create an unintended deployment risk. Common causes include a worn under-seat wiring harness or an internal short in the pretensioner actuator. Stop driving and visit an authorised dealer for inspection and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal 6 EV
  • 1Worn wiring harness under the seat: Frequent fore-and-aft adjustment of the front passenger seat wears the insulation of the pretensioner wiring harness (usually routed from the B-pillar through the seat slide rail). The internal copper wire contacts the vehicle body metal frame, creating a short to ground.
  • 2Pretensioner actuator internal short circuit: The ignition bridge wire inside the second-stage pretensioner shorts to the housing, or moisture in the propellant increases conductivity, usually causing abnormal resistance (<0.5Ω).
  • 3Connector water ingress and corrosion: Driving through water or cleaning the interior allows water to enter the yellow SRS connector under the front passenger seat (usually located at the front of the seat rail), causing a short circuit between terminals or a terminal short to ground.
  • 4SRS control unit internal drive circuit fault: The ignition driver chip inside the control module (such as NXP MC33780) breaks down, causing continuous voltage output at the pretensioner power supply terminal or an internal short circuit.
  • 5Wiring modification interference: Non-OEM modifications (such as seat heating pads or massage modules) incorrectly connected to the SRS circuit, or seat mounting screws pinching the wiring harness and causing insulation damage.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal, wait at least 90 seconds to fully discharge the SRS capacitor, wear an anti-static wrist strap, and place high-voltage warning signs around the front passenger seat.
  • 2
    Freeze frame analysis: Use the BYD VDS2000/Launch X431 diagnostic tool to read the fault freeze frame. Record the vehicle speed, seat position, and ambient temperature at the time of the fault. Determine whether the fault is continuous or intermittent.
  • 3
    Visual inspection: Remove the front passenger seat (mark all connectors before disconnecting). Inspect the yellow SRS connector under the seat (usually B11 or D07) for water ingress, terminal oxidation, or burning. Check the wiring harness for wear at the seat slide rail retaining clips.
  • 4
    Resistance measurement: Disconnect the SRS control unit connector (located under the center console or inside the center armrest box). Use a digital multimeter to measure the resistance between the front passenger pretensioner second-stage circuit terminals (usually pins 15 and 16). The normal value is 2.0-3.0Ω. Measure the resistance from each terminal to ground. The value must be >1MΩ.
  • 5
    Wiring harness insulation test: Use an insulation resistance tester (megohmmeter) to apply 500V DC to the wiring harness and check for a short to ground or short to power. Focus inspection on wiring harness bends within the seat travel range.
  • 6
    Component replacement verification: If the wiring harness is normal, replace the front passenger seat belt assembly (including the pretensioner). Verify the new part resistance value is within the specified range. If the fault persists after replacing the pretensioner, replace the SRS control unit.
  • 7
    System Calibration: After repair, connect the diagnostic tool and perform 'SRS System Configuration' (enter the correct vehicle VIN and airbag configuration code). Perform 'Pretensioner Resistance Learning' and 'System Self-check'. Verify no current fault codes exist and stored fault codes clear successfully.
  • 8
    Function verification: Restore all connections. Turn on the power and observe if the airbag warning lamp turns off after 6 seconds. Use the diagnostic tool to perform the 'Passive Safety System Function Test'. Simulate a crash signal to verify the pretensioner circuit response (read the real-time resistance value without disconnecting the connector).
Real-World Cases
BYD DTC AI Analysis

Worn front passenger seat wiring harness causing intermittent B179B fault on Song MAX

✓
A 2020 Song MAX, 32,000 km, instrument cluster intermittently displayed "Check SRS System." Scan tool retrieved intermittent DTC B179B. Inspection found the front passenger seat rail rear mounting bolt pinching the wiring, chafing the pretensioner power supply insulation and partially exposing the copper. With the seat in specific positions, the exposed harness shorted to ground. Repair: Repaired the damaged wiring with double-layer heat-shrink tubing, rerouted the harness clear of the seat rail mounting point, and replaced the mounting bolt with a dedicated part featuring a wire channel. No recurrence after three months.
BYD DTC AI Analysis

Tang DM-i front passenger pretensioner internal short circuit caused airbag warning light to stay on.

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2022 BYD Tang DM-i. Passenger airbag warning light stayed on after driving through water. Read DTC B179B current fault. Disassembled and inspected; found obvious water stains and verdigris on the SRS connector beneath the passenger seat, but cleaning did not clear the fault. Measured pretensioner resistance at only 0.3Ω (normal 2.5Ω). Diagnosed water ingress into the second-stage pretensioner causing a short in the igniter bridge wire. Replaced the OEM passenger seatbelt assembly (part number: SA-5820100), dried the wiring harness connector, and applied conductive grease. Fault resolved. Replacement of a safety component requires registration with the local vehicle authority.
BYD DTC AI Analysis

Yuan EV SRS control unit internal fault misdiagnosed as pretensioner failure

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2019 BYD Yuan EV535. DTC B179B set after accident repair (front impact). The shop had replaced the passenger seatbelt assembly but the fault remained. Further inspection revealed collision impact damaged the SRS control unit (located beneath the gear selector), burning out the internal ignition driver chip. We measured 12V continuously at the control unit output terminals in the untriggered state, causing the system to detect a short to power. Replaced the SRS control unit (part number: BYD-3638100) and recoded/configured the airbag system (requires online configuration using a BYD factory account). Fault resolved. Distinguish between a "wiring short" and "control unit false alarm" on these.
BYD DTC AI Analysis

Aftermarket seat ventilation installation caused pretensioner wiring interference on Qin PLUS DM-i

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2021 BYD Qin PLUS DM-i. After the customer retrofitted a third-party ventilation and heating system to the front passenger seat, DTC B179B set while driving. Inspection found the installer had spliced the ventilation system power line into the SRS pretensioner supply (yellow harness), causing abnormal voltage in the pretensioner circuit. The control unit detected unexpected voltage fluctuations and diagnosed a power short. Repair: Removed the unauthorized wiring and restored factory harness insulation via soldering and heat-shrink tubing. Replaced the contaminated pretensioner connector (terminals deformed). Note: Never connect any loads in parallel to SRS circuits. The system demands strict line impedance tolerance (±0.5Ω).
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.