AU
B179C

DTC B179C indicates an abnormally low-resistance connection between the ignition circuit of the front passenger-side seat belt second stage pretensioner and body ground — Atto 3

Safety System

DTC B179C indicates an abnormally low-resistance connection between the ignition circuit of the front passenger-side seat belt second stage pretensioner and body ground.

In the BYD SRS (Supplemental Restraint System) architecture, a twisted pair typically connects the second stage pretensioner to the Airbag Control Unit (ACU), with a normal resistance of approximately 2.0–3.0 Ω.

The ACU sets this DTC when it detects the circuit-to-ground resistance falls below the threshold (typically <1.0 Ω).

This fault causes: 1) unintentional pretensioner deployment in non-collision situations, causing personal injury; 2) failure of the pretensioner to deploy correctly during a collision; 3) the ACU to enter fault protection mode, which may subsequently disable the front passenger airbag and side curtain airbags, greatly reducing crash safety.

This is a hard fault; once confirmed, it continuously illuminates the SRS warning lamp.

The B179C fault code on the Atto 3 indicates a high-severity short to ground in the front passenger seat belt second stage pretensioner circuit. This can affect SRS operation and may disable key airbags or prevent correct pretensioner deployment. Common causes include worn under-seat wiring or a pinched harness near the B-pillar trim. Visit an authorised dealer for diagnosis and wiring 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
  • 1Worn under-seat wiring harness: Long-term friction from the front passenger seat fore-and-aft adjustment mechanism damages the pretensioner wiring harness insulation, allowing the copper core to contact the seat rail or body metal and create a short to ground.
  • 2Pinched wiring harness inside B-pillar trim: Improperly securing the harness during B-pillar interior trim removal and installation or body panel repair allows trim clips or metal edges to crush the harness, causing a short to ground.
  • 3Connector terminal corrosion due to water ingress: Vehicle wading or a blocked sunroof drain causes water to accumulate on the front passenger floor. The seal on the pretensioner connector (usually located under the seat or at the base of the B-pillar) fails, forming an electrolytic corrosion path between the terminals.
  • 4Pretensioner internal fault: The bridge wire of the internal squib shorts to the housing. This typically occurs after a severe collision or due to a manufacturing defect (low occurrence rate, but requires inspection).
  • 5SRS control unit internal drive circuit fault: ACU internal ignition drive transistor breakdown or ground monitoring circuit abnormality causing a false code (verify by substitution).
What To Do
  • 1
    Safety preparation: Switch off the ignition, disconnect the negative battery terminal and wait at least 90 seconds (to fully discharge the SRS capacitor). Wear an anti-static wrist strap. Do not use radio equipment near the airbag/pretensioner.
  • 2
    Fault Confirmation and Freeze Frame Analysis: Use a dedicated BYD diagnostic tool (VDS or ED400) to read the B179C freeze frame data. Record the vehicle speed, temperature, and voltage at the time of the fault. Confirm whether the fault is intermittent or continuous.
  • 3
    Visual inspection and wiring harness check: Inspect the wiring harnesses under the front passenger seat, at the base of the B-pillar, and along the floor. Focus on contact points between the wiring harness and the seat slide rails or metal brackets. Look for wear, damaged insulation, and crush marks. Inspect connectors for looseness or water ingress.
  • 4
    Circuit isolation test: Disconnect the front passenger second-stage pretensioner connector (yellow connector, usually under the seat or at the B-pillar). Use a multimeter to measure resistance to ground on the wiring harness side. If resistance is <5Ω, this indicates a wiring harness short to ground. If resistance is infinite, measure resistance at the pretensioner end. Normal resistance is 2.0±0.5Ω. If <1Ω or infinite, replace the pretensioner.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it using heat-shrink tubing or waterproof tape. Re-route the harness to maintain a clearance of more than 20mm from metal components. Replace damaged retaining clips and install abrasion-resistant sleeving if necessary.
  • 6
    Component replacement: If the pretensioner body fails, replace the front passenger seat belt assembly (the seat belt retractor usually integrates the pretensioner). Verify normal resistance before installing the new part. Never measure the resistance of an installed pretensioner (this may trigger ignition).
  • 7
    System verification: Restore all connections, turn on power, clear fault codes, and perform SRS system self-test (turn key to ON position and verify warning light turns off after 6 seconds). Use the diagnostic tool to execute the 'Collision Output Check' function (Simulation Test) to confirm B179C does not return.
  • 8
    Final check: Reinstall the seat and interior trim, perform a road test, and verify the seat adjustment function does not interfere with the wiring harness. Confirm no abnormalities exist before returning the vehicle.
Real-World Cases
BYD DTC AI Analysis

Seat rail chafed wiring harness, causing intermittent short circuit

✓
A 2021 BYD Tang DM with 32,000 km showed an intermittent 'SRS Fault' warning. The scanner read B179C as a history fault. Inspection found the front passenger seat pretensioner harness chafing against the slide rail edge when the seat moved. The worn insulation exposed the copper core, which contacted the rail and shorted to ground at specific seat positions. Repaired the harness with flame-retardant corrugated tubing sleeving, rerouted it, and secured it with OEM retaining clips to maintain 30 mm clearance from the slide rail. Fault resolved.
BYD DTC AI Analysis

Corroded connector caused short circuit after water ingress

✓
2019 BYD Yuan EV. SRS warning light stayed on after rainwater leaked into the front passenger floor through a blocked front windscreen scuttle drain. DTC B179C present. Removed and inspected the yellow pretensioner connector beneath the front passenger seat; found obvious water stains inside with terminals oxidised green. Measured terminal-to-ground resistance at only 0.3Ω. Repair: Thoroughly cleaned the connector terminals with electronic cleaner, dried with compressed air, applied conductive grease, replaced the waterproof seal, and performed an insulation test on the wiring harness. Cleared the scuttle drain. Fault resolved.
BYD DTC AI Analysis

B-pillar wiring harness incorrectly fitted after accident repair

✓
2020 BYD Qin Pro (fuel). One week after right-front collision repairs, the airbag warning light illuminated. Diagnostics showed DTCs B179C and B179B (power short circuit). Removing the B-pillar trim revealed the pretensioner harness had been left unclipped from its plastic sheath during the accident repair; the B-pillar sheet metal edge crushed the wiring, internally shorting both power and ground to the body. Replaced the damaged harness section (floor to top of B-pillar), rerouted and secured with OEM clips. Cleared the fault codes; system normal.
BYD DTC AI Analysis

Internal short circuit in pretensioner body caused hard fault

✓
2019 BYD Song MAX, no collision history. SRS warning light suddenly illuminated and would not clear; DTC B179C present. Disconnected the passenger pretensioner connector. Measured infinite resistance from the harness side to ground (normal). Measured 0.1Ω between the pretensioner terminals and housing (abnormal). Confirmed the pretensioner internal igniter shorted to housing. Replaced the passenger seatbelt assembly (including pretensioner). New part measured 2.3Ω before installation. After installation, cleared the fault code and the system passed self-test.
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.