AU
B1771A

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

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.

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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Wiring harness wear under the right rear seat or inside the B-pillar trim panel: Frequent fore/aft seat movement or passengers stepping on the harness damages the insulation, shorting the positive and negative wires and causing an abnormal resistance drop.
  • 2Pretensioner connector water ingress and oxidation: Vehicle wading, a blocked sunroof drain tube, or liquid seepage during deep interior cleaning causes a short circuit or abnormal resistance between 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: Damage to the control unit's internal sampling resistor or A/D conversion circuit produces a false low-resistance signal.
  • 5Improper modification or accident repair: The seat frame pinches the wiring harness, or the connector is not fully locked when retrofitting seat heating, replacing the carpet, or performing post-accident repairs.
What To Do
  • 1
    Safety preparation: Use the 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 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor.
  • 2
    Locate the component: Remove the right rear B-pillar lower trim panel (or seat outer trim panel, depending on vehicle structure) and locate the seat belt pretensioner assembly, 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 harness-side insulation resistance to ground and to power. The resistance must be greater than 1 MΩ; otherwise, repair the wiring harness.
  • 5
    Circuit continuity check: Disconnect the SRS ECU connector (usually under the center console or in the front compartment). Measure the 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 dangerous goods transport and storage regulations. During replacement, never measure the new part directly with a multimeter resistance setting; use a dedicated diagnostic tool to activate and test it.)
  • 7
    System reset: After repair, reconnect all connectors and the 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 normally.
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 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.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.