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 — Seal U

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 — Seal U
  • 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 (typically >2Ω).
  • 2Wiring harness worn by seat adjustment mechanism: Frequent fore-and-aft sliding or folding of the middle-row seat breaks the internal copper strands of the pretensioner wiring harness at the hinge (broken core, intact insulation), creating a high-resistance circuit.
  • 3Pretensioner unit aging: Long-term electrolysis from monitoring micro-currents or material fatigue increases the resistance of the gas generator squib bridge wire, approaching the open-circuit threshold.
  • 4Modification or repair damage: fixing screws piercing the wiring harness during installation of seat ventilation/heating pads or leather trim, or connectors not fully seated into 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 (perform a left-right swap test to rule out).
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal. Wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment. Set up high-voltage safety warnings (for new energy vehicles, confirm high-voltage system de-energization).
  • 2
    Visual inspection: Remove the left middle row seat side trim panel or the lower B-pillar trim panel. Verify the yellow pretensioner connector (usually marked 'SRS' or 'Pretensioner') is fully seated. Inspect the terminals for green copper 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 1.6-3.0 Ω. If the reading is >3.5 Ω or 'OL' (open circuit), proceed to the next step.
  • 4
    Harness continuity test: Disconnect the pretensioner and SRS module connectors. Measure continuity between both ends of the harness; resistance must be <1 Ω. Also measure harness insulation resistance to ground; resistance must be >1 MΩ (to rule out voltage division caused by 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 severed wiring harnesses and apply double insulation (heat-shrink tubing + electrical tape). Replace a faulty pretensioner with an OEM part (match the part number; resistance specifications may differ by model year).
  • 7
    System verification: Restore all connections, power on, and clear the fault code. Perform the SRS system self-check (this usually requires cycling the ignition switch ON-OFF 3 times). Confirm B1779 does not return and the airbag warning light turns off. Finally, perform a seat sliding test to verify no wiring harness interference.
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 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.