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 8

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 8 indicates a medium-severity short circuit in the second-row left seat belt pretensioner circuit, with resistance near 0Ω instead of normal. Common causes include damaged wiring shorting to body ground under the seat or near the B-pillar, or a corroded/bent pretensioner connector. Visit an authorised dealer for SRS diagnosis and repair. Real-case typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Pretensioner wiring harness shorted to ground due to wear: Long-term fore-and-aft seat adjustment and vibration friction damage the harness insulation under the left middle-row seat or near the B-pillar, allowing the wire to contact the vehicle body metal and create a short circuit.
  • 2Pretensioner connector internal short circuit: Bent or backed-out connector pins, or water ingress and corrosion (common after vehicle wading or interior cleaning) causing continuity between terminals.
  • 3Pretensioner assembly internal short circuit: Squib internal bridge wire blown or chemically deteriorated, causing a sharp drop in resistance, or pretensioner deployed and not reset.
  • 4SRS control unit internal detection circuit fault: Abnormal ECU internal sampling resistor or A/D conversion circuit causes false detection (less common; rule out external wiring before confirming).
  • 5Improper modification or repair: Retaining screws pierce the wiring harness when installing seat covers or seat heating pads, or when removing and 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 battery negative terminal, and wait at least 90 seconds for the SRS capacitor to fully discharge, preventing accidental deployment.
  • 2
    Visual inspection and connector check: 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 at the seat slide rail fixing points for wear or cuts.
  • 3
    Pretensioner body resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter set to resistance to measure the resistance directly across 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
    Harness insulation test: Keep the pretensioner disconnected. Measure the resistance from both terminals on the harness side of the connector to body ground. Resistance must be >1MΩ. If continuity exists, inspect the 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 transfers to B17781A (right middle row). If the code transfers, the original pretensioner has an intermittent short circuit. If the code does not transfer, check the wiring harness at the SRS ECU.
  • 6
    Repair and replacement: Repair the damaged wiring harness (use heat-shrink tubing for double-layer insulation). Replace the short-circuited pretensioner (use genuine parts; 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. Use VDS or a dedicated diagnostic tool to clear the fault code. Execute SRS system self-learning (if applicable). Perform a vehicle collision simulation test (use the diagnostic tool to execute the component test function and observe the data stream to verify the resistance returns to a normal value of approximately 2.0Ω). Confirm the SRS warning light on the instrument cluster 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.