AU
B1789

On BYD vehicles, DTC B1789 indicates a circuit fault in the driver seat belt pretensioner or the seat belt buckle position sensor, specifically a short to ground, open circuit, or signal range/performance fault — Atto 3

Safety System

On BYD vehicles, DTC B1789 indicates a circuit fault in the driver seat belt pretensioner or the seat belt buckle position sensor, specifically a short to ground, open circuit, or signal range/performance fault.

Although early documentation may describe this as a 'second-stage front airbag', the fault code points to the seat belt restraint system components beneath the seat.

The SRS control module triggers B1789 when it detects abnormal resistance in the driver-side seat belt pretensioner circuit (below 1.0Ω or above 4.0Ω; normal range is 2.0-3.0Ω) or when the buckle position sensor signal voltage remains below the threshold.

This fault forces the airbag system into fail-safe mode.

In a collision, the system may fail to accurately determine if the driver is wearing the seat belt, preventing intended airbag deployment, disabling the pretensioner, or triggering start inhibition logic on certain models.

The fault also illuminates the instrument cluster SRS warning light continuously, indicating a substantial risk to the occupant protection system.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Wiring harness under the driver's seat worn or crushed: Frequent forward and backward seat adjustment causes the seat belt pretensioner wiring harness to interfere with the seat rail, resulting in damaged insulation, broken copper wires, or a short circuit.
  • 2Poor contact at the seat belt pretensioner connector (yellow plug): plug not fully locked, internal pins oxidized, pins backed out, or water ingress corrosion, resulting in abnormal resistance.
  • 3Seat belt pretensioner unit fault: Pretensioner deployed after a vehicle accident (micro gas generator exploded) without replacement during repair, causing an internal open circuit; or pretensioner internal coil short circuit.
  • 4Seat anchorage position sensor detached or damaged: Failure to reconnect the sensor connector after seat removal and installation, or a broken sensor retaining clip causing signal interruption.
  • 5Wiring harness retaining clip failure: A broken retaining clip allows the wiring harness to chafe against the metal body bracket over bumps, causing an intermittent short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the 12 V battery negative terminal and wait at least 3 minutes to fully discharge the SRS system capacitors and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use VDS or the BYD dedicated diagnostic tool to read the fault code, confirm whether B1789 is a current (Active) or history (History) fault, and record the freeze frame data.
  • 3
    Visual inspection: Move the driver's seat to the rearmost position. Inspect the wiring harness under the seat for signs of abrasion, pinching, or damaged insulation. Focus on interference points between the seat rails and the wiring harness.
  • 4
    Connector inspection: Disconnect the seat belt pretensioner connector (usually yellow). Check the plug for full engagement and inspect the pins for oxidation, burning, or backed-out terminals. Clean with electrical contact cleaner if necessary.
  • 5
    Resistance measurement: Use a multimeter to measure the pretensioner unit resistance. The normal value is 2.0-3.0 Ω. Infinite resistance indicates an open circuit (pretensioner deployed or wiring harness broken). Zero or extremely low resistance indicates a short to ground.
  • 6
    Wiring harness repair: If the wiring harness is damaged, strip the insulation, solder the break, and wrap it in layers using heat-shrink tubing or insulating tape. Reroute the harness and secure it with cable ties. Maintain a clearance of more than 20 mm from moving parts.
  • 7
    Component replacement: If the pretensioner is faulty or has deployed, replace the entire driver seat belt assembly (including the pretensioner). Do not repair the pretensioner separately.
  • 8
    System reset: Connect the battery, clear the fault code, perform 'SRS system configuration' or 'sensor calibration' (if applicable), perform a static self-test and road test, and confirm the fault light turns off and the fault does not return.
Real-World Cases
BYD DTC AI Analysis

Song MAX seat rail pinched seatbelt pretensioner wiring harness

✓
A 2019 BYD Song MAX suddenly displayed "Please check SRS system" on the dashboard while driving. VDS retrieved DTC B1789. Technicians moved the seat fully rearward and found the seat rail interfering with the seatbelt pretensioner wiring harness. The rail had crushed through the insulation and partially broken the copper wires, causing an intermittent open circuit. They soldered the break, applied insulation and waterproofing, rerouted the harness, and secured it to the seat frame with cable ties away from the rail. Clearing the fault codes resolved the issue.
Original source ↗
BYD DTC AI Analysis

Qin Plus DM-i seat belt pretensioner connector loose and oxidised

✓
A Qin Plus DM-i had the airbag warning light on and chiming after a cold start. VDS detected DTC B1789. Found the yellow connector under the driver's seat not fully locked (knocked loose during previous interior cleaning) with minor oxidation on the internal pins. Repair: Disconnected the battery, unplugged the connector, and cleaned the pins with electronic contact cleaner. Reseated it firmly and secured it with a cable tie. Cleared the fault code. System returned to normal.
Original source ↗
BYD DTC AI Analysis

Unreplaced pretensioner caused open circuit after S6 accident repair

✓
After a left-front collision repair, a BYD S6 displayed an SRS warning. DTC B1789 would not clear. The driver seatbelt pretensioner measured open circuit (infinite resistance). Disassembly revealed the pretensioner had deployed in the accident (gas generator exploded), but the initial repair only replaced body panels, not the seatbelt assembly. Fix: Replaced the driver seatbelt assembly (including pretensioner and anchorage position sensor), performed 'SRS configuration write' and 'sensor calibration' with a diagnostic tool. Fault resolved.
Original source ↗
BYD DTC AI Analysis

F3: Anchor position sensor connector unplugged after seat removal and refitting.

✓
A BYD F3 owner removed the driver's seat to install seat covers. After reinstalling it, the airbag warning light stayed on. The scanner showed DTC B1789. Inspection found the seat belt anchor position sensor connector under the seat had been left unplugged; the system detected an open circuit. Repair: Reconnected the sensor connector until it clicked, cleared the DTC, and the system returned to normal.
Original source ↗
BYD DTC AI Analysis

Han EV intermittent B1789 wiring harness chafing on rough roads

✓
A Han EV intermittently showed an SRS warning on rough roads, sometimes clearing after a restart. History stored DTC B1789 (intermittent). Inspection revealed a broken under-seat wiring harness retainer clip; the harness was rubbing against a metal body bracket, causing an intermittent short to ground. Repaired the harness by re-wrapping with insulation tape, replaced the retainer clip to ensure safe clearance, and applied conductive grease at the connector to prevent oxidation. The fault has not returned.
Original source ↗
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.