AU
B179A

DTC B179A indicates the SRS (Supplemental Restraint System) ECU detects an open circuit or disconnected front passenger-side second-stage seat belt pretensioner — Atto 3

Safety System

DTC B179A indicates the SRS (Supplemental Restraint System) ECU detects an open circuit or disconnected front passenger-side second-stage seat belt pretensioner.

The "second stage" typically refers to the retractor pretensioner (distinct from the first-stage buckle pretensioner), which uses a pyrotechnic igniter to rapidly tighten the seat belt during a collision.

The SRS ECU monitors the pretensioner circuit resistance (normally 2.0-4.0 Ω) to determine connection status.

The ECU sets this DTC when resistance exceeds the threshold (typically >10 kΩ or a complete open circuit).

This fault prevents pretensioner activation during a collision, reducing chest and head restraint protection for the front passenger, and illuminates the instrument panel airbag warning light.

Because occupants of various body sizes frequently adjust the front passenger seat, this fault occurs significantly more often in shared vehicles than on the driver side.

The B179A fault code on the Atto 3 indicates an open or disconnected circuit in the front passenger-side second-stage seat belt pretensioner. This is a medium-severity SRS fault because the pretensioner may not activate in a collision and the airbag warning light may come on. Common causes include a loose under-seat connector or an internal pretensioner igniter fault. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Loose or disconnected wiring harness connector under the front passenger seat (frequent fore/aft seat adjustment wears the connector locking tab or strains the wiring harness)
  • 2Pretensioner gas generator internal open circuit (igniter aging, moisture ingress, or broken internal bridge wire causing infinite resistance)
  • 3Wiring harness inside the B-pillar trim worn and broken (long-term friction from the mechanical movement of seat belt extension and retraction causes copper wire fatigue and breakage)
  • 4Water ingress or high humidity causes connector terminal oxidation and corrosion (ECU identifies increased contact resistance as an open circuit).
  • 5Pretensioner connector not fully seated after accident repairs or interior trim removal and installation, or damaged connector latch causing poor contact.
What To Do
  • 1
    Safety preparation: For new energy vehicles, first disconnect the high-voltage service disconnect. For all vehicles, disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Inspection procedure: Remove the front passenger-side B-pillar lower trim panel (remove the seat on some models). Locate the yellow second-stage pretensioner wiring harness connector. Verify it is fully inserted and listen for a locking click. Confirm the waterproof sealing ring is intact.
  • 3
    Resistance measurement: Use a digital multimeter to measure the resistance between the two pretensioner terminals. The normal value is 2.0-4.0 Ω. If the multimeter displays OL (open circuit) or >10 Ω, disconnect the connector and measure the pretensioner body directly to determine whether the fault is in the pretensioner or the wiring harness.
  • 4
    Harness continuity test: Check wiring harness continuity from the SRS ECU (usually located in the lower center console) to the pretensioner connector. Focus on areas prone to pinching or abrasion, specifically near the seat rail and B-pillar bend.
  • 5
    Cleaning and protection: If the connector shows oxidation or corrosion, clean it with electrical contact cleaner and apply specialized conductive protective grease. Replace the connector housing if necessary to ensure reliable locking.
  • 6
    System reset: Reconnect all components and the battery, use the BYD VDS diagnostic tool to clear the fault code, and perform the SRS system self-check cycle (ignition ON, wait for the system self-check to complete).
  • 7
    Function verification: View the 'Front passenger second-stage pretensioner resistance' live value in the data stream and confirm it displays 2-4Ω (green normal range). Perform a simulated crash test or road test to verify the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Frequent seat adjustment loosened the connector

✓
A 2021 BYD Tang DM with 32,000 km presented with an intermittently illuminated airbag warning light. A VDS scan returned DTC B179A-00 (current fault). Inspection revealed the front passenger seat pretensioner connector lock tab had fractured beneath the seat; frequent adjustment by different occupants had caused the connector to work loose. Replaced the connector with an OEM unit, rerouted the wiring harness, and secured it with cable ties to prevent movement with the seat. Fault permanently resolved.
BYD DTC AI Analysis

Connector oxidised and corroded after water ingress

✓
2019 BYD Yuan EV. Airbag warning light stayed on after wading through approximately 30 cm of water. Scan tool showed B179A. Removed the passenger-side B-pillar trim and found water staining and green copper corrosion inside the pretensioner connector. Terminal oxidation caused contact resistance exceeding 50 Ω. Cleaned the terminals with precision electronic cleaner, applied conductive protective grease, and replaced the waterproof seal ring. Advised the owner to inspect the door seals and avoid driving through water.
BYD DTC AI Analysis

Open circuit in pretensioner gas generator

✓
2020 BYD Qin Pro, no collision history. Dashboard suddenly displayed airbag fault. Measured passenger-side second-stage pretensioner resistance as infinite (OL). Disassembly found open circuit in pretensioner's internal igniter (internal fault). Replaced OEM seatbelt assembly (including pretensioner), used diagnostic tool to perform 'Replace Airbag ECU/Component' coding and matching procedure, cleared fault codes, system returned to normal.
BYD DTC AI Analysis

Aftermarket seats crushed the wiring harness

✓
2019 Song MAX. Owner fitted aftermarket electric seats, triggering DTC B179A. Inspection found seat rail mounting bolts had crushed the pretensioner harness during modification, severing internal copper strands while outer insulation appeared undamaged. Stripping back the wiring revealed three copper conductors completely broken. Repaired the damaged harness, rerouted clear of seat moving mechanisms, and protected with corrugated tubing. Fault cleared and owner counselled regarding modification risks.
BYD DTC AI Analysis

B-pillar trim incorrectly fitted, crushed wiring harness

✓
2021 Tang. Passenger airbag warning light on after accident repair. Found DTC B179A. Removed B-pillar trim and found the wiring harness misrouted during the previous repair; trim clips were pressing directly on the pretensioner harness, nearly severing it. Replaced the damaged harness section, rerouted per standard procedure, and ensured adequate slack for seatbelt travel. Fault resolved.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.