AU
B1641

DTC B1641 indicates an abnormally low-resistance path to body ground (short circuit) in the driver-side seat belt pretensioner ignition circuit — Atto 3

Safety System

DTC B1641 indicates an abnormally low-resistance path to body ground (short circuit) in the driver-side seat belt pretensioner ignition circuit.

In the BYD SRS (Supplemental Restraint System) architecture, the pretensioner is a pyrotechnic safety device with a normal squib resistance of approximately 2.0-3.0 Ω.

The SRS control unit triggers this DTC when it detects the circuit resistance to ground falls below the threshold (typically <1.0 Ω).

This fault forces the SRS into fail-safe mode: the airbag warning lamp remains illuminated, and the fault may completely disable the system.

During a collision, the pretensioner fails to deploy and retract the seat belt, severely threatening occupant safety.

Furthermore, a continuous short to ground can overheat and damage the internal driver circuit of the SRS control unit, expanding the repair scope.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Worn insulation on the pretensioner wiring harness under the driver's seat contacts the metal seat frame or slide rail, causing a short to ground. This commonly occurs in vehicles with frequent seat adjustments or detached harness retaining clips.
  • 2Water ingress into the yellow under-seat SRS connector (usually on the inner side of the seat rail), or corroded, bent, or deformed pins, causes a short circuit between the signal wire and ground. This frequently occurs after driving through water or deep interior cleaning.
  • 3A manufacturing defect or aging of the seat belt pretensioner internal igniter causes an internal short to ground. This commonly occurs in aftermarket parts used during accident repairs or components degraded by prolonged high-temperature exposure.
  • 4SRS control unit internal detection circuit or driver transistor breakdown falsely reports a short to ground. Other SRS fault codes usually set simultaneously.
  • 5Damage to the vehicle floor wiring harness sheath where it passes through the firewall or seat crossmember causes wires to contact body metal. This commonly results from chassis bottoming out or incorrect harness routing during seat modification.
What To Do
  • 1
    Safety preparation: Disconnect the negative battery terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental deployment. Before disconnecting, record radio presets and one-touch window positions.
  • 2
    Fault confirmation: Connect the VDS or dedicated diagnostic tool, confirm DTC B1641 is Active, and record the ambient temperature and operating voltage from the Freeze Frame data.
  • 3
    Initial inspection: Remove the driver seat (keep the wiring harness connected). Locate the yellow SRS connector (usually marked DAB or Pretensioner). Measure the resistance between the pretensioner terminals (normal: 2.0-3.0 Ω) and the resistance from each terminal to ground (should be >1 MΩ).
  • 4
    Wiring harness physical inspection: Inspect the wiring harness sleeve behind and on the inner side of the seat slide rail for damage. Check the seat height adjustment motor harness and pretensioner harness for interference wear. Check the wiring harness retaining clips for detachment.
  • 5
    In-depth connector inspection: Disconnect the connector and inspect the pins for oxidation, backed-out pins, or signs of electrolyte corrosion. Measure continuity to ground on the harness side of the connector. If necessary, clean with electrical contact cleaner and blow dry.
  • 6
    Component isolation test: Disconnect the pretensioner connector and connect a 2-3Ω dedicated shorting resistor (load tool) in place of the pretensioner. If the fault code changes to open circuit (B1640/B1643), this indicates an internal short circuit in the pretensioner assembly. Replace the pretensioner assembly.
  • 7
    Harness continuity test: If the system still reports a short to ground after installing the shorting resistor, measure the resistance to ground of the wiring between the connector and the SRS control unit. Check the intermediate adapter connector (usually located under the carpet or in the A-pillar) for water ingress.
  • 8
    Control unit check: After confirming no short circuit in the wiring harness, measure the resistance to ground at the corresponding pin of the SRS control unit. If abnormal, replace the SRS ECU. After replacement, perform VIN writing, configuration coding, and crash sensor calibration.
  • 9
    Repair verification: Clear the fault code after repair and perform an SRS system self-check (cycle ignition 3 times and confirm the warning lamp turns off after the 6-second self-check). Perform a full seat travel adjustment test (fore/aft, up/down, backrest angle) to verify no interference.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Tang DM: Worn seat wiring harness causing intermittent B1641 fault

✓
Symptoms: The airbag warning light on the instrument panel illuminates intermittently, becoming more frequent on rough roads. Diagnosis: Retrieved current DTC B1641 using VDS. Removed the driver's seat and found the wiring harness boot damaged behind the seat rail; copper wires contacting the seat frame caused an intermittent short circuit. Measured pretensioner circuit resistance to earth: 0.5–2.0 ohms fluctuating. Repair: Repaired the damaged wiring using wire rated for temperatures above 150°C, rerouted the harness to avoid the seat rail travel area, and fitted corrugated tubing and cable ties. Cleared the DTC and cycled the seat adjustment 200 times continuously with no recurrence.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Song MAX Pretensioner Connector Corrosion Case After Wading Through Water

✓
Symptoms: After driving through water, the airbag warning light illuminated on the instrument cluster. A VDS scan revealed DTC B1641. Diagnosis: Following the body domain controller water ingress troubleshooting procedure, we inspected the floor wiring harness for water ingress. We found significant water staining and verdigris inside the yellow SRS connector beneath the driver's seat. Resistance between pin 3 (pretensioner high) and pin 4 (earth) measured 0.8 Ω. Repair: Cleaned the connector with isopropyl alcohol, dried the harness, applied conductive grease (not insulating silicone), and replaced the connector seal. This fixed the fault and the SRS system passed its self-test.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket pretensioner short circuit after accident repair on Qin Pro

✓
Symptoms: Following front collision repairs, the instrument panel airbag warning light remained constantly illuminated, with VDS reporting DTC B1641. Diagnosis: Disconnected the pretensioner connector and measured infinite resistance from the harness to ground (normal). Measured the resistance of the newly fitted aftermarket pretensioner at 0.2Ω, indicating an internal short to ground. Repair: Replaced the pretensioner assembly with a genuine unit. Pretensioners are single-use pyrotechnic components. Dispose of the old unit as hazardous waste and never measure resistance across the terminals. After installing the new component, measured resistance at 2.3Ω (normal). Cleared the DTC and performed a simulated crash test using the diagnostic tool actuator test function. The system returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Tang DM SRS Control Unit Internal Fault Misdiagnosis Case

✓
Symptoms: DTC B1641 remained after replacing the pre-tensioner and wiring harness, accompanied by multiple SRS communication faults. Diagnosis: Referring to the body domain controller troubleshooting case for 'network paralysis causing multi-system faults', we measured the SRS ECU to pre-tensioner circuit resistance to earth and found it normal (>1MΩ). We determined that an internal driver circuit breakdown in the SRS control unit was causing a short to earth. Repair: Replaced the SRS control unit, wrote the VIN, configured the system, and calibrated the crash sensors. Executed 'Configuration Setting' and 'Longitudinal Acceleration Sensor Learning' using the diagnostic tool. This completely resolved the fault.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Yuan EV seat modification pinched the wiring harness, causing a short circuit.

✓
**Symptoms:** The airbag warning light came on after the owner modified the electric seat. **Diagnosis:** Following the diagnostic approach for blind spot monitor (BSM) 'incorrect installation position' faults, we found the seat frame pinched the pretensioner harness during the modification, damaging the insulation and causing a short to ground. **Repair:** Replaced the damaged harness section using OEM wire with matching colour and gauge, rerouted the harness, and kept at least 20mm clearance from the seat frame. Cleared the fault codes and tested the seat through its full range of movement; the fault did not return.
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. Sources: [1]