AU
B179B

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

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.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Under-seat wiring harness wear: Frequent fore-and-aft adjustment of the front passenger seat wears the insulation on the pretensioner wiring harness (usually routed from the B-pillar through the seat track), causing the internal copper wire to contact the vehicle metal frame and create 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 short to ground.
  • 4SRS control unit internal drive circuit fault: Electrical breakdown of the ignition driver chip (e.g., NXP MC33780) inside the control module causes continuous voltage output at the pretensioner power supply terminal or an internal short circuit.
  • 5Aftermarket wiring 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 damaging the insulation.
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. Confirm whether the fault is persistent or intermittent.
  • 3
    Visual inspection: Remove the front passenger seat (mark all connectors before disconnecting). Closely inspect the yellow SRS connector (usually B11 or D07) under the seat for water ingress, terminal oxidation, or burning. Inspect 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 2.0-3.0 Ω. Measure the resistance from each terminal to ground. The value must be >1 MΩ.
  • 5
    Harness insulation test: Use an insulation resistance tester (megohmmeter) to apply 500V DC to the harness and check for a short to ground or power. Focus on bent harness sections within the seat movement 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 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 execute "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 clear stored fault codes.
  • 8
    Function verification: Reconnect all connections. Power on and verify the airbag warning light turns off after 6 seconds. Use the diagnostic tool to perform the 'Passive Safety System Function Test'. Simulate a collision 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.

✓
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

✓
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

✓
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 U 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.