AU
B1781

DTC B1781 indicates the airbag system (SRS) detected an abnormal short to vehicle power (B+, typically 12V battery voltage) in the right middle-row seat belt pretensioner ignition circuit — Atto 3

Safety System

DTC B1781 indicates the airbag system (SRS) detected an abnormal short to vehicle power (B+, typically 12V battery voltage) in the right middle-row seat belt pretensioner ignition circuit.

The pretensioner is an electrically triggered ignition device containing an igniter and a gas generator.

Under normal conditions, the circuit remains in a high-resistance open state (showing a low resistance of 2-3Ω only when measured with a dedicated tool).

When the Airbag Control Unit (ACU) detects the circuit voltage continuously exceeding the threshold (typically above 5V) or an abnormal resistance drop, it identifies a short to power.

This fault forces the SRS into fail-safe mode, disabling the right middle-row and potentially related airbag functions.

The airbag warning light illuminates continuously.

In severe cases, the pretensioner may fail to deploy during a collision or may trigger unintentionally.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Physical damage to the wiring harness: Long-term friction during seat track adjustment damages the insulation of the harness under the right middle-row seat, inside the B-pillar trim panel, or the floor harness. This causes a short circuit where the pretensioner power wire contacts the body power wire or a constant power circuit.
  • 2Connector fault: Water ingress, oxidation, bent pins, or backed-out pins in the pretensioner connector under the seat (usually located on the seat frame or base of the B-pillar) cause adjacent terminals (power and signal/ground) to short circuit.
  • 3Pretensioner internal short circuit: Internal component breakdown in the seat belt retractor assembly pretensioner igniter (such as a squib short circuit) causes resistance to drop abnormally close to 0 Ω.
  • 4SRS control unit (ACU) fault: Damaged ACU internal drive circuit or monitoring chip triggers a false code (confirm after ruling out wiring harness and components).
  • 5Aftermarket modifications: When installing seat heating, ventilation, or massage functions, the installer incorrectly connected the pretensioner wiring harness in parallel with the vehicle constant power supply, or improperly secured the harness, allowing the seat rail to crush and damage the wires.
What To Do
  • 1
    Safety preparation and diagnostic confirmation: Use the BYD dedicated diagnostic tool (VDS or ED400) to read the fault code. Confirm B1781 is a current fault (Active Code) and not a history fault. Record the freeze frame data. Disconnect the 12V battery negative terminal and wait at least 3 minutes for the SRS capacitor to fully discharge.
  • 2
    Visual inspection and connector check: Remove the right middle-row seat (or lower B-pillar trim panel). Inspect the pretensioner connector (usually a yellow plug) for looseness, water ingress, corrosion, or physical damage. Check for bent, backed-out, or bridged pins. Clean the connector with compressed air and apply special conductive grease.
  • 3
    Circuit continuity and insulation test: Use a multimeter to measure the resistance between the pretensioner terminals (normal: 2.0-3.0 Ω; <1.0 Ω indicates an internal short circuit); measure the resistance between the terminals and ground (must be >1 MΩ); measure the voltage at the terminals (must be <1 V; 12 V confirms a short to power); use a probe to check harness continuity, focusing on the harness sheath near the seat rail and the B-pillar hinge.
  • 4
    Wiring harness repair or component replacement: If the wiring harness is damaged, repair it with heat-shrink tubing or waterproof tape. Reroute the harness to avoid interference with seat mechanical components. If the pretensioner itself has a short circuit, replace the right second-row seat belt assembly (including the pretensioner). Do not repair the pretensioner separately. If the ACU is faulty, replace the airbag control unit and perform coding configuration.
  • 5
    System reset and function verification: Reconnect all connectors and the battery, then clear the fault codes; execute the SRS system self-test (ACU initialization); perform a static test (simulate a crash signal to test circuit continuity without triggering the pretensioner); confirm the airbag warning light turns off within 6 seconds; perform a road test, read the live data stream using the diagnostic tool, and confirm the pretensioner circuit resistance and voltage are within the normal range.
Real-World Cases
BYD DTC AI Analysis

Song MAX right second-row seat slide rail wear caused wiring harness short circuit

✓
2019 Song MAX, 30,000 km. Airbag warning light stayed on. Retrieved DTC B1781. Found: Right middle seat slide rail pinches seatbelt pretensioner harness during fore-aft adjustment. Insulation worn through, shorting to frame (ground) and constant power. Replaced the damaged harness section (~30 cm), added corrugated tubing, and repositioned clips to maintain 15 mm clearance from rail. Fixed.
BYD DTC AI Analysis

Tang DM seat modification caused pretensioner short to power

✓
The owner of a 2021 Tang DM reported the airbag warning light illuminated with DTC B1781. Upon inquiry, the owner had recently had middle-row seat heating installed at an aftermarket workshop. Inspection revealed the installer had mistakenly connected the seat heater power wire to the pretensioner connector, causing the pretensioner circuit to connect directly to 12V power. Repair: Removed unauthorised modification wiring, restored factory harness connections, replaced damaged pretensioner connector terminals, and re-wrapped the harness. Advised the owner that seat modifications must use dedicated power supply interfaces.
BYD DTC AI Analysis

Yuan EV pretensioner connector corroded and shorted after water ingress

✓
2019 Yuan EV set B1781 after wading through heavy rain. Found the pretensioner connector beneath the right middle-row seat had water stains and green copper corrosion; terminals were shorted by electrolyte conduction. Cause: Wading depth exceeded the floor seal line and water entered through the B-pillar seal. Repair: Dried the connector thoroughly, cleaned terminals with precision electronic cleaner, applied dielectric grease (insulating silicone grease), and replaced the waterproof seal ring. Check other SRS connectors for water ingress.
BYD DTC AI Analysis

Qin Pro seatbelt pretensioner internal squib short circuit

✓
2020 Qin Pro petrol variant showed DTC B1781 that would not clear. After checking the wiring, the harness between the ACU and pretensioner showed good insulation with no short to power. Disconnected the pretensioner connector and measured the body resistance: 0.3Ω (well below the normal 2.3Ω). Determined the internal igniter element had shorted. Fix: Replaced the right middle-row seat belt assembly (part number: BYD-Safety-RC-2020), ran the 'Replace Airbag Control Unit/Component' programming routine on the diagnostic tool, and wrote the system configuration.
BYD DTC AI Analysis

Multiple models: B-pillar trim panel installed improperly, pinching wiring harness

✓
Multiple Yuan and Song series vehicles developed DTC B1781 after B-pillar trim repairs (such as window glass replacement or seat belt height adjuster repairs). Cause: When reinstalling the lower B-pillar trim, technicians failed to seat the pretensioner wiring harness correctly in the wire channel, causing trim clips to press directly on the harness. Prolonged vibration pierced the insulation, allowing contact with the metal trim clips (connected to body power). Repair: Repair the damaged harness, reroute the wiring to specification, and ensure the harness sits in the dedicated wire channel clear of trim compression.
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.