AU
B1771

DTC B1771 indicates the SRS (Supplemental Restraint System) control module detects the right rear seat belt pretensioner resistance is below the system-calibrated lower threshold (typically <1 — Atto 3

Safety System

DTC B1771 indicates the SRS (Supplemental Restraint System) control module detects the right rear seat belt pretensioner resistance is below the system-calibrated lower threshold (typically <1.0Ω or near a short-circuit value).

The pretensioner is a pyrotechnic actuator containing an electric squib with a normal resistance of approximately 2.0±0.3Ω.

Low resistance indicates a pretensioner internal short circuit, a wiring harness short to ground, connector water ingress causing electrochemical corrosion, or an SRS module internal sampling circuit fault.

This fault triggers the airbag system fail-safe strategy.

The affected seat pretensioner may fail to deploy in a collision, the AIRBAG warning lamp remains illuminated, and some models trigger a system buzzer alarm.

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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Internal short circuit in the right rear seat belt pretensioner: The bridge wire inside the igniter breaks and shorts to ground, or moisture in the pyrotechnic charge causes an abnormal drop in resistance.
  • 2Wiring harness wear causing short to ground: The wiring harness below the B-pillar or the floor wiring harness rubs against the metal body frame, damaging the insulation and causing a short circuit between the positive and negative terminals or a short to ground.
  • 3Connector water ingress and oxidation: A flooded rear floor, degraded seals, or water seepage during a car wash introduces water into the pretensioner connector (typically located below the B-pillar or beside the seat). This causes electrochemical corrosion of the terminals and creates a low-resistance path.
  • 4SRS control module internal fault: ECU internal sampling resistor fault, A/D conversion circuit fault, or abnormal software calibration data causes a false low resistance report.
  • 5Improper modification or repair: Tools piercing the wiring harness or failing to fully seat connectors when installing seat covers, modifying seat heating, or removing and installing the seat causes a short circuit between terminals.
What To Do
  • 1
    Safety Preparation and Diagnostic Confirmation: Use BYD VDS2000 or Launch X431 to read all DTCs. Confirm B1771 is an active fault, not a historical fault. Record freeze frame data (vehicle status, mileage, voltage). Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS energy storage capacitor.
  • 2
    Visual inspection and connector check: Remove the right rear lower B-pillar trim panel and sill trim panel. Inspect the pretensioner connector (usually a yellow connector with a short-circuit protection tab) for looseness, water ingress, corrosion, or backed-out pins. Inspect the wiring harness jacket for abrasion, burn marks, or rodent bite marks.
  • 3
    Resistance measurement and component isolation: Disconnect the pretensioner connector. Use a digital multimeter (minimum 10 MΩ input impedance) to measure the resistance between the two terminals on the pretensioner body. Standard value: 2.0 ± 0.3 Ω. If <1.0 Ω, replace the pretensioner assembly. If normal, measure the resistance to ground at the two terminals on the wiring harness side. The resistance must be >10 MΩ; otherwise, inspect the wiring harness for wear points.
  • 4
    Harness repair and insulation test: If the harness is damaged, repair the area with two layers of high-temperature insulation tape (Tesa 51036 or equivalent). Replace the harness section if necessary. After repair, measure harness continuity and insulation resistance to ground and power. Verify no short circuit risk.
  • 5
    SRS Module Inspection: If the pretensioner and wiring harness are normal, check the SRS module connector pins (usually beneath the centre tunnel) for oxidation or push-back. Use an oscilloscope to check the pretensioner drive circuit waveform. Update the SRS module software or replace the control unit if necessary.
  • 6
    Component replacement and system configuration: When replacing a faulty pretensioner, use only genuine parts. Do not measure the resistance of the new or old part, as the multimeter current may trigger the squib. After replacement, perform 'Airbag System Configuration Write' (enter the VIN and vehicle configuration code) and clear the DTC.
  • 7
    System self-check and verification: Connect the battery and turn the key to the ON position. Verify the instrument cluster airbag warning light turns off after 6 seconds. Use the diagnostic tool to perform the 'SRS System Active Test' (if supported) and confirm the system has no communication faults and generates no new DTCs. Perform a road test to verify the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Repaired short circuit in right rear seat belt pretensioner on water-damaged vehicle

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Vehicle: 2020 BYD Song Pro DM. Fault: After heavy rain, the airbag warning light stayed on constantly. DTC B1771 (current). Found the right rear carpet wet. Removed the B-pillar trim panel and found obvious water inside the pretensioner connector; the pins were oxidized and blackened. Cleaned the connector with alcohol and dried it. The pretensioner body measured normal resistance (1.9Ω), but resistance between the connector terminals was only 0.3Ω. Replaced the wiring harness connector and waterproofed it. Fault cleared. Recommend checking the sunroof drain hoses and door seals.
BYD DTC AI Analysis

Seat modification caused wiring harness wear.

✓
Model: 2019 Yuan EV535. Airbag warning light illuminated after fitting leather seat covers. Diagnosis revealed intermittent DTC B1771. Removed the right rear seat and found the installer had accidentally driven securing pins through the pretensioner wiring harness when fitting the seat covers, shorting the positive and negative wire cores. Repaired the damaged harness (2 wires, 0.5mm²) and rerouted it to avoid the seat rail travel path. Fault resolved. Note: Disconnect the battery and mark pretensioner harness positions when removing or fitting seats.
BYD DTC AI Analysis

Replaced pretensioner due to internal short circuit

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Model: 2021 Tang DM-i. Symptom: No collision history, but airbag warning lamp suddenly illuminated while driving; DTC B1771 present. Disconnected pretensioner connector and measured unit resistance at only 0.6Ω—far below specification. Diagnosis: Internal pretensioner igniter short. Repair: Replaced right rear seatbelt assembly (pretensioner integrated with retractor), performed system configuration write. Fault resolved. Findings: Disassembly of old part revealed internal propellant had absorbed moisture and caked, causing bridge wire to short against housing.
BYD DTC AI Analysis

Software fix for SRS module false positive

✓
Vehicle: 2019 Qin Pro EV. Fault: DTC B1771 occurs intermittently, with increased frequency during humid weather. Diagnosis: Measured pretensioner and wiring harness resistance within specification; inspected connectors with no faults found. Reviewed technical bulletins revealing an SRS software calibration defect in this production batch causing false low-resistance readings during voltage fluctuations. Repair: Upgraded SRS control module software to V2.3.5 or above. Result: Fault has not reoccurred. Software updates are critical for diagnosis.
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.