AU
B17771A

DTC B17771A indicates the airbag control unit (SRS ECU) detects a circuit resistance of 0Ω or close to 0Ω for the second-row left seat belt pretensioner — Atto 3

Safety System

DTC B17771A indicates the airbag control unit (SRS ECU) detects a circuit resistance of 0Ω or close to 0Ω for the second-row left seat belt pretensioner.

Normally, the squib resistance inside an undeployed pretensioner is 2.0Ω±0.2Ω.

A resistance of 0Ω indicates a hard short in the circuit.

Potential causes include an internal short in the pretensioner coil, a wiring harness short to body ground, or a short between connector terminals.

This is a safety system hard fault.

The SRS ECU illuminates the airbag malfunction indicator lamp (MIL) and disables the second-row left seat belt pretensioner and potentially associated airbag functions.

During a collision, the pretensioner fails to generate the designed pulling force to tighten the seat belt, reducing occupant restraint protection.

The B17771A fault code on the Atto 3 indicates a medium-severity SRS fault where the second-row left seat belt pretensioner circuit is shorted, showing 0Ω or near 0Ω resistance. Common causes include damaged wiring shorting to body ground or corrosion, bent pins, or water ingress in the pretensioner connector. The seat belt pretensioner may not work in a crash. 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
  • 1Pretensioner wiring harness wear causing a short to ground: Long-term fore-and-aft seat adjustment and vibration friction damage the wiring harness insulation under the left middle-row seat or near the B-pillar. The exposed wire contacts the vehicle body metal, creating a short circuit.
  • 2Internal short circuit in the pretensioner connector: bent pins, backed-out pins, or water ingress and corrosion (common after vehicle wading or interior cleaning) causing continuity between terminals.
  • 3Pretensioner body internal short circuit: The bridge wire inside the squib blew or chemically deteriorated, causing a sharp drop in resistance, or the pretensioner deployed and remains unreset.
  • 4SRS control unit internal detection circuit fault: Abnormal ECU internal sampling resistor or A/D conversion circuit causing false detection (less common; confirm only after ruling out external wiring).
  • 5Improper modification or repair: Retaining screws pierce the wiring harness when installing seat covers or seat heating pads, or when removing/installing interior trim, causing a direct short circuit between the positive and negative terminals.
What To Do
  • 1
    Safe power-off and discharge: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Visual and connector inspection: Remove the left middle-row seat side trim panel. Inspect the pretensioner connector (usually yellow) for looseness, water ingress, or deformed pins. Inspect the wiring harness for wear or cuts at the seat slide rail fixing points.
  • 3
    Pretensioner body resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter set to resistance to measure the resistance directly between the pretensioner terminals. The normal value is 1.8-2.2 Ω. If the reading is 0 Ω or <1 Ω, replace the pretensioner assembly.
  • 4
    Wiring harness insulation check: Keep the pretensioner disconnected. Measure the resistance between both terminals on the harness side of the connector and body ground. Resistance must be >1 MΩ. If continuity exists, inspect the wiring harness section by section, focusing on the inside of the harness sleeve under the seat.
  • 5
    Swap verification test (optional): If resistance is normal, swap the left middle-row and right middle-row pretensioner connectors. Check if the fault code shifts to B17781A (right middle row). If it shifts, confirm an intermittent short circuit in the original pretensioner. If it does not shift, check the wiring harness at the SRS ECU end.
  • 6
    Repair and replacement: Repair the damaged wiring harness using double-layer heat-shrink tubing. Replace the short-circuited pretensioner with a genuine part (do not measure new part resistance to prevent accidental deployment). Verify the connector waterproof sealing ring is intact.
  • 7
    System reset and verification: Connect the battery. Clear the fault code using VDS or a dedicated diagnostic tool. Execute SRS system self-learning (if applicable). Perform a vehicle crash simulation test (use the diagnostic tool to execute the component test function and verify the data stream resistance value returns to normal, approximately 2.0 Ω). Confirm the instrument cluster SRS warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Worn seat rails caused the wiring harness to short to ground

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Vehicle: 2020 BYD E3, 32,000 km. Symptom: SRS warning light steady on, DTC B17771A stored. Found the code set intermittently when sliding the left middle row seat fore and aft. Removed the seat; the pretensioner harness had chafed against the seat rail mounting bracket, wearing through the insulation and shorting the copper conductors to ground. Repaired the harness with heat-shrink tubing, rerouted the wiring and added chafe protection. Fault resolved.
BYD DTC AI Analysis

Corroded connectors caused short circuit after water wading

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Vehicle: 2019 Qin EV, previously waded through water up to sill height. Fault: Airbag warning light on; diagnostic tool showed left middle row pretensioner resistance at 0 ohms. Repair: Found visible water staining and green copper corrosion inside the yellow connector beneath the left middle row seat, with a conductive layer between terminals causing a short. Cleaned the connector terminals, flushed with electronic contact cleaner and dried, applied conductive protectant, and reinstalled. Recommend inspecting the sealing integrity of other SRS connectors on the vehicle.
BYD DTC AI Analysis

Seatbelt pretensioner squib internal short circuit

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Vehicle: 2020 E2, accident-repaired. Fault: DTC B17771A after left middle-row seat replacement. Diagnosis: New seat pretensioner resistance measured 0.2Ω (abnormal). Disassembly found a refurbished pretensioner with partially triggered squib causing internal short. Repair: Installed genuine new pretensioner assembly (part number must match VIN). Fault cleared. Note: Accident repairs require untriggered genuine pretensioners.
BYD DTC AI Analysis

Incorrect wiring of aftermarket seat heating pads caused the fault.

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Vehicle: 2019 Qin EV, aftermarket modified. Symptom: DTC B17771A appeared after seat heater installation. Repair: The installer drilled through the seat foam to secure the heating pad, driving the screw straight through the pretensioner harness and shorting positive and negative. Replaced the damaged harness section and repositioned the heating pad to clear the factory wiring. Fault resolved. Note: SRS modifications must strictly avoid yellow wiring harnesses.
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.