AU
B1779

DTC B1779 indicates the SRS (Supplemental Restraint System) control module detected the second-row left seat belt pretensioner circuit resistance exceeds the calibrated threshold (typically >3 — Atto 3

Safety System

DTC B1779 indicates the SRS (Supplemental Restraint System) control module detected the second-row left seat belt pretensioner circuit resistance exceeds the calibrated threshold (typically >3.5-4.0Ω; normal range is 1.6-3.0Ω).

This passive safety system circuit integrity fault indicates a high-resistance state in the pretensioner deployment circuit.

Potential causes include poor connections, an open circuit in the wiring harness, or an aging gas generator coil inside the pretensioner.

During a collision, the SRS module might not supply sufficient current to deploy the pretensioner, preventing the seat belt from tightening promptly and increasing the risk of occupant injury.

The system also illuminates the airbag warning lamp and may disable the associated airbag protection functions for that seat.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Pretensioner wiring harness connector loose or oxidized: Water ingress, cleaning fluid penetration, or long-term vibration oxidizes the terminals of the pretensioner connector under the left middle-row seat or at the B-pillar, creating contact resistance (usually >2Ω).
  • 2Seat adjustment mechanism wearing the wiring harness: Frequent forward and backward sliding or folding of the middle-row seat breaks the internal copper strands of the pretensioner wiring harness at the hinge (core broken but insulation intact), creating a high-resistance circuit path.
  • 3Pretensioner unit aging: Gas generator internal squib bridge wire resistance increases due to material fatigue or long-term electrolysis caused by diagnostic micro-currents, approaching the open-circuit threshold.
  • 4Modification or repair damage: Fixing screws pierced the wiring harness during the installation of seat ventilation/heating pads or leather upholstery, or the connector did not fully engage the locked position after accident repairs.
  • 5SRS control module sampling error: Fault in the ACU (Airbag Control Unit) internal A/D conversion circuit or voltage divider resistor causes a false high resistance reading (rule out via left-right swap test).
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal. Wait at least 90 seconds for the SRS capacitor to discharge fully and prevent accidental airbag deployment. Set up high-voltage safety warnings (for new energy vehicles, confirm high-voltage system shutdown).
  • 2
    Visual inspection: Remove the left middle-row seat side trim panel or lower B-pillar trim panel. Verify the yellow pretensioner connector (usually marked 'SRS' or 'Pretensioner') is fully inserted. Inspect the terminals for green corrosion, burn marks, or backed-out pins.
  • 3
    Resistance measurement: Use a dedicated low-current SRS multimeter (test current <10mA) to measure the resistance across the pretensioner terminals. The normal value is 1.6-3.0Ω. If the reading is >3.5Ω or 'OL' (open circuit), proceed to the next step.
  • 4
    Harness continuity test: Disconnect the pretensioner connector and the SRS module connector. Measure continuity between both ends of the harness. Resistance should be <1Ω. Also measure harness-to-ground insulation resistance. Resistance should be >1MΩ (to rule out voltage division from a short to ground).
  • 5
    Cross-check: Swap the left and right second-row pretensioner connectors (if vehicle configuration is symmetrical). If the fault code moves with the component to B177A (right second-row resistance too high), replace the pretensioner assembly. If the fault code remains in the original position, inspect and repair the wiring harness or SRS module.
  • 6
    Repair and replacement: Clean oxidised connectors with electrical contact cleaner. Solder broken wiring harnesses and apply double insulation (heat-shrink tubing + insulating tape). Replace a faulty pretensioner with a genuine part (match the part number; resistance specifications may differ between model years).
  • 7
    System verification: Restore all connections, power on, and clear the fault code. Perform an SRS system self-check (usually requires cycling the ignition switch ON-OFF 3 times). Verify B1779 does not return and the airbag warning light turns off. Finally, perform a seat sliding test to verify the wiring harness does not interfere.
Real-World Cases
BYD DTC AI Analysis

Internal open circuit in Song MAX middle row seat wiring harness

✓
A 2019 Song MAX with 80,000 km had the airbag warning light on. Scanning showed DTC B1779. Inspection found the wiring harness under the left middle-row seat had intact outer insulation but 90% of the copper conductors inside were broken at the slide-rail hinge, with only a few strands left, causing 3.8 Ω resistance. Cause: The owner frequently folded the middle-row seats completely flat to carry cargo, repeatedly forcing the wiring into acute-angle bends. Repair: Cut out the damaged section, soldered extension wires using flexible silicone cable, and added harness retaining clips to avoid bend radius under 50 mm. Fault resolved.
BYD DTC AI Analysis

Tang DM connector oxidised after water ingress.

✓
2021 Tang DM. DTC B1779 with B1778 (low resistance) intermittent. History of B-pillar water ingress from blocked sunroof drains. Found severe terminal oxidation (green) and a broken locking tab on the left middle-row pretensioner connector (inside C-pillar trim), causing poor contact. Replaced the entire pretensioner harness connector (with terminals), treated with DeoxIT Gold, applied conductive paste and wrapped in waterproof tape. Cleared the sunroof drains. Resistance stabilised at 2.1Ω after repair.
BYD DTC AI Analysis

BYD Yuan EV connector loose after accident repair

✓
2019 Yuan EV. B1779 set three days after right-front collision repairs. Inspection found the left middle-row seatbelt pretensioner connector (under the seat frame) wasn't fully latched—inserted but the lock never clicked. Vibration caused contact resistance to fluctuate between 2.5–5Ω. Cause: The technician failed to verify the connector's secondary lock during the accident repair (some BYD models use a two-stage locking design). Fix: Reseated the plug until the white locking tab popped up, then secured the harness with a cable tie to eliminate strain. Cleared the DTC; no recurrence.
BYD DTC AI Analysis

Qin Pro pretensioner body aged, high resistance

✓
2020 BYD Qin Fuel, 120,000 km. Persistent DTC B1779. Measured pretensioner resistance at 4.2Ω, unstable (fluctuated when wiggling the harness), but harness continuity was normal. Swapped left and right middle row pretensioners; fault transferred to the right side (B177A). Diagnosed as internal gas generator squib aging. Repair: Replaced left middle row seatbelt assembly (with pretensioner), new part resistance 2.0Ω. Analysis: Continuous micro-current monitoring in high-mileage vehicles may cause electrolytic aging of the squib bridge wire; recommend direct replacement for vehicles over 100,000 km exhibiting this fault.
BYD DTC AI Analysis

Wiring harness accidentally pierced during aftermarket seat ventilation installation

✓
Case: Yuan EV 2019. Owner installed a middle-row seat ventilation system at an auto parts market; the airbag warning light came on the next day with DTC B1779. Inspection found a self-tapping screw securing the fan had pierced the left middle-row seat pretensioner harness when the installer drilled the foam. The copper strands were partially broken but not fully severed. Resistance measured 3.6Ω. Fix: Removed the seat ventilation kit. Cut out the damaged section, soldered the wires, and insulated with dual-wall heat shrink tubing. Relocated the fan mount to clear the factory harness. Reminder: BYD routes middle-row seat pretensioner wiring inside the seat frame. Check the wiring diagram before modifying.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.