AU
B1771A

DTC B1771A indicates the SRS (Supplemental Restraint System) ECU detects the RH rear seatbelt pretensioner circuit resistance is below the calibrated threshold — Atto 3

Safety System

DTC B1771A indicates the SRS (Supplemental Restraint System) ECU detects the RH rear seatbelt pretensioner circuit resistance is below the calibrated threshold. (The standard value is typically 2.0 ± 0.5 Ω; a measured value below 1.2 Ω or near 0 Ω triggers this code).

As a key passive safety system actuator, the pretensioner contains a pyrotechnic combustion chamber and a resistance wire.

Low resistance indicates a short circuit risk (harness chafing to ground, water ingress at the connector, or an internal short from a melted resistance wire).

To prevent accidental deployment or upon determining actuator failure, the SRS ECU illuminates the airbag warning light and may disable the RH rear side airbag and pretensioner deployment.

This disables active tensioning protection for that seat during a collision, increasing occupant injury risk.

The B1771A fault code on the Atto 3 indicates the SRS ECU has detected low resistance in the right rear seatbelt pretensioner circuit. Severity is medium because the airbag warning light may come on and pretensioner protection for that seat may be disabled. Common causes include damaged wiring under the rear seat or water/oxidation in the pretensioner connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Wiring harness wear under the right rear seat or inside the B-pillar trim: Frequent fore/aft seat movement or passengers stepping on the harness damages the insulation, shorting the positive and negative wires and causing an abnormal drop in resistance.
  • 2Pretensioner connector water ingress and oxidation: Driving through water, a blocked sunroof drain tube, or liquid seepage during deep interior cleaning causes a short circuit or abnormal resistance between the connector terminals.
  • 3Pretensioner unit internal fault: Aging, moisture ingress, or manufacturing defects cause an inter-turn short circuit in the internal resistance wire of the pyrotechnic device, resulting in permanently low resistance.
  • 4SRS ECU detection circuit fault: A damaged internal sampling resistor or A/D conversion circuit in the control unit produces a false low-resistance signal.
  • 5Improper modification or accident repair: Seat frame pinches the wiring harness or connector fails to lock fully when retrofitting seat heaters, replacing the carpet, or performing post-accident repairs.
What To Do
  • 1
    Safety preparation: Use a BYD VDS2000 or Launch X431 diagnostic tool to read the fault code. Confirm B1771A is a current fault, not a history fault, and record the freeze frame data. Turn off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds for the SRS capacitor to fully discharge.
  • 2
    Locate the component: Remove the right rear B-pillar lower trim panel (or seat outer trim panel, depending on vehicle structure) and find the seat belt pretensioner assembly. This is typically a black cylindrical pyrotechnic device with a yellow two-color connector.
  • 3
    Visual inspection: Check the pretensioner connector for looseness, the terminals for green oxidation, and the wiring harness for damage or crush marks. Focus on the wiring harness sleeve at the seat slide rail mounting points.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Set a multimeter to the 200 Ω range and measure the resistance between the pretensioner-side terminals. The standard value is 1.8-2.5 Ω. If the resistance is below 1.2 Ω, replace the pretensioner. Measure the insulation resistance from the harness side to ground and to the power supply. The resistance must exceed 1 MΩ; otherwise, repair the wiring harness.
  • 5
    Circuit continuity check: Disconnect the SRS ECU connector (usually located under the center console or in the front compartment). Measure harness continuity between the pretensioner connector and the ECU. Confirm no short to ground or short between wires.
  • 6
    Component replacement: If the pretensioner is faulty, replace it with a genuine part. (Note: The pretensioner is an explosive component; follow hazardous materials transport and storage regulations. During replacement, never measure the new part directly using a multimeter resistance setting. Activate and test the part using a dedicated diagnostic tool.)
  • 7
    System reset: After repair, reconnect all connectors and the 12V battery. Use the diagnostic tool to clear the fault code. Perform the 'SRS System Self-Learning' or 'Configure New Sensor' function. Confirm the fault code does not return and verify the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Worn seat rails shorted the wiring harness

✓
A 2019 Qin PRO DM with 32,000 km had the airbag warning light on. Diagnostic scanner reported DTC B1771A. Found: wiring harness beneath right rear seat chafed against slide rail mounting bracket, insulation worn through causing positive-to-negative short, resistance measured at 0.8Ω. Repaired harness, installed abrasion-resistant corrugated tubing, rerouted clear of moving parts. Fault resolved.
BYD DTC AI Analysis

Connector corrosion after water ingress caused fault

✓
2018 BYD Qin Pro petrol. Owner reported the airbag warning light stayed on after driving through deep water. Inspection found water staining and corrosion inside the right rear pretensioner connector, with insulation resistance between terminals measuring only 200Ω. Water entered through the door sill trim during wading due to a degraded connector seal. Repair: Cleaned the connector terminals, applied electronic contact cleaner, and fitted a replacement waterproof connector. Fault resolved.
BYD DTC AI Analysis

Connector not locked after accident repair

✓
A Qin PRO DM had its right rear seatbelt assembly replaced after collision repair, but the airbag warning light came on after delivery. DTC B1771A. The pretensioner connector was inserted but the locking tab hadn't clicked into place. Driving vibration caused intermittent contact resistance at the terminals (intermittent low resistance). The fault cleared after reinserting the connector firmly and confirming the locking tab was fully seated.
BYD DTC AI Analysis

Pretensioner internal resistance wire short circuit

✓
2019 BYD Qin Pro, no accidents or water ingress. DTC B1771A appeared suddenly. Disconnected the harness and measured the pretensioner body resistance directly: 0.6Ω. Internal resistance wire short—likely a batch quality defect. Replaced the right rear seatbelt pretensioner assembly (requires replacement with the seatbelt; pretensioners not available separately). Configured the new part and cleared the fault.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.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.
Common problems reported for Atto 3 →Atto 3 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.