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 8

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.

The B1779 fault code on the Atto 8 indicates a medium-severity SRS fault where the second-row left seat belt pretensioner circuit has higher resistance than expected. Common causes include a loose or oxidized pretensioner connector, or seat movement damaging the wiring harness near the seat hinge. This may stop the pretensioner working in a crash. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Pretensioner wiring harness connector loose or oxidized: Water ingress, cleaning fluid penetration, or prolonged 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 (broken core with intact insulation), creating a high-resistance path.
  • 3Pretensioner unit aging: Material fatigue or long-term micro-current electrolysis from current monitoring increases resistance in the gas generator squib bridge wire, approaching the open-circuit threshold.
  • 4Modification or repair damage: Fixing screws pierced the wiring harness during installation of seat ventilation/heating pads or leather trim, or the connector is not fully seated in the locked position after accident repairs.
  • 5SRS control module sampling error: A fault in the ACU (Airbag Control Unit) internal A/D conversion circuit or voltage divider resistor causes a false high resistance reading (rule out by performing a left-right swap test).
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds for the SRS capacitor to discharge fully to prevent accidental airbag deployment. Set up high-voltage safety warnings (for new energy vehicles, confirm the high-voltage system is powered off).
  • 2
    Visual inspection: Remove the left middle-row seat side trim panel or lower B-pillar trim panel. Check that the yellow pretensioner connector (usually marked 'SRS' or 'Pretensioner') is fully seated. Inspect the terminals for green corrosion, burn marks, or backed-out pins.
  • 3
    Resistance measurement: Use a dedicated low-current SRS multimeter (test current <10 mA) to measure the resistance across the pretensioner terminals. Normal value is 1.6-3.0 Ω. If the reading is >3.5 Ω or "OL" (open circuit), continue 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 must be <1Ω. Measure the harness insulation resistance to ground. Resistance must 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 transfers 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 oxidized connectors with electrical contact cleaner. Solder broken wiring harnesses and apply double insulation (heat-shrink tubing + electrical tape). Replace a faulty pretensioner assembly 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 the SRS system self-check (usually requires cycling the ignition switch ON-OFF 3 times). Confirm B1779 does not reappear 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.