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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 — Qin Plus

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.

The B1789 fault code on the Qin Plus indicates a high-severity fault in the driver seat belt pretensioner or buckle position sensor circuit. It may be caused by damaged wiring under the driver’s seat or poor contact at the yellow pretensioner connector. The SRS warning light may stay on and occupant protection may be reduced. Visit an authorised BYD dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Worn, crushed, or broken wiring harness under the driver's seat: 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): incompletely locked plug, oxidized internal pins, backed-out pins, or water ingress corrosion, causing abnormal resistance.
  • 3Seat belt pretensioner unit fault: Pretensioner deployed after a vehicle accident (micro gas generator exploded) but not replaced during repair, causing an internal open circuit; or pretensioner internal coil short circuit.
  • 4Seat anchorage position sensor detached or damaged: Sensor connector left disconnected after seat removal and installation, or broken sensor retaining clip causing signal interruption.
  • 5Harness retaining clip failure: A broken harness retaining clip allows the wiring harness to rub against the metal body bracket over bumps, causing an intermittent short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 3 minutes for the SRS system capacitors to fully discharge to prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use VDS or a dedicated BYD 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 terminals for oxidation, burning, or backed-out pins. Clean with electrical contact cleaner if necessary.
  • 5
    Resistance measurement: Use a multimeter to measure the pretensioner unit resistance. The normal value is 2.0-3.0Ω. 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 at least 20mm from moving parts.
  • 7
    Component replacement: If the pretensioner is faulty or deployed, replace the complete 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), conduct a static self-check and road test, and verify the warning 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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.