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 — Seal U

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 — Seal U
  • 1Worn, crushed, or broken wiring harness under the driver's seat: Frequent fore-aft seat adjustment causes the seat belt pretensioner 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, causing abnormal resistance.
  • 3Seat belt pretensioner unit fault: Pretensioner deployed after a vehicle accident (micro gas generator detonated) and failure to replace it during repair caused an internal open circuit; or the pretensioner internal coil short-circuited.
  • 4Seat anchorage position sensor detached or damaged: Failure to connect the sensor connector after removing and installing the seat, or a broken sensor retaining clip causing signal interruption.
  • 5Wiring harness retaining clip failure: A broken retaining clip causes the wiring harness to rub against the metal body bracket over bumps, resulting in an intermittent short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal. Wait at least 3 minutes for the SRS system capacitors to fully discharge to 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 fault (Active) or a history fault (History), 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, crushing, 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). Verify the plug is fully locked. Inspect the pins for oxidation, burning, or backing out. Clean with electrical contact cleaner if necessary.
  • 5
    Resistance measurement: Use a multimeter to measure the pretensioner 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, maintaining a clearance of at least 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), and conduct a static self-check and road test to verify the warning light turns off and the fault does not recur.
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 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.