AU
B17721B

DTC B17721B indicates the airbag system (SRS) detects the right rear seat belt pretensioner circuit resistance exceeds the normal upper threshold — Atto 3

Safety System

DTC B17721B indicates the airbag system (SRS) detects the right rear seat belt pretensioner circuit resistance exceeds the normal upper threshold.

This pretensioner is a pyrotechnic actuator with a typical resistance range of 2.0–3.0 ohms.

When the ECU detects resistance >4.0 ohms (or per the calibrated threshold), it determines a High Resistance fault.

This fault prevents the seat pretensioner from deploying and retracting normally during a collision.

Additionally, the SRS may enter a degraded mode, illuminating the instrument cluster airbag warning light continuously.

On some models, other seat airbag functions may also be restricted.

The DTC suffix "1B" typically indicates the specific circuit location or fault subtype, pointing to the right rear seat (Third Row/Second Row Right) pretensioner circuit.

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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Poor connector contact: Moisture, oxidation, or vibration increases pin contact resistance at the right B-pillar or under-seat pretensioner connector (usually marked R2R or SA3). This is the most common cause, occurring frequently after water wading or vehicle washing.
  • 2Harness damage causing high resistance: Crushing the harness during rear seat removal/installation, carpet modification, or child seat installation partially breaks the internal copper strands without fully severing them, creating a high-resistance point.
  • 3Pretensioner internal fault: Squib bridge wire oxidation or breakage, or moisture ingress into the pyrotechnic charge, causes resistance to drift upward over time, indicating actuator aging failure.
  • 4Ground point corrosion: Electrochemical corrosion at the pretensioner circuit ground point (usually located on the right rear longitudinal beam or C-pillar) increases ground resistance, creating high series resistance.
  • 5SRS ECU internal sampling circuit fault: A faulty sampling resistor or A/D conversion circuit on the corresponding channel within the airbag control module triggers a false code (relatively rare; consider only after ruling out wiring issues).
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds (3 minutes for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use a dedicated diagnostic tool (such as Launch X431 or BYD VDS) to read the fault code. Confirm B17721B is a current fault (Present) and not a history fault. Record the resistance value in the freeze frame data (usually displaying 4.5-10 ohms or Open).
  • 3
    Visual inspection: Remove the right rear seat. Inspect the pretensioner connector (yellow plug, usually located on the inner side of the seat or below the B-pillar) for looseness, water ingress, or oxidized pins. Inspect the wiring harness for crush marks from the seat rail.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter (low-current range) to measure resistance directly across the pretensioner terminals. The normal value is 2.0 ± 0.5 ohms. If resistance is >3.5 ohms, replace the pretensioner assembly.
  • 5
    Circuit continuity test: If the pretensioner is normal, measure the circuit resistance between the connector and the SRS ECU; resistance should be <1 ohm. Measure the insulation resistance to ground; resistance should be >10M ohms. Inspect the wiring harness for internal breaks at the B-pillar pass-through.
  • 6
    Repair Verification: After repairing or replacing the faulty component, reconnect the battery, clear the fault code, and perform an SRS system self-check (turn the ignition switch ON and verify the airbag warning light turns off after 6 seconds). If necessary, perform a simulated crash test (use a dedicated resistor substitute to verify circuit integrity).
Real-World Cases
BYD DTC AI Analysis

Right rear seat removal and reinstallation pinched the wiring harness, causing high resistance.

✓
Vehicle: Qin Pro DM 2019, 32,000 km. Customer reported constant airbag warning light. Retrieved DTC B17721B. Inspection revealed right rear seat upholstery removed and reinstalled at a workshop one week prior. Disassembly showed pretensioner harness pinched between seat frame and slide rail; insulation damaged but copper strands not fully severed. Resistance measured 5.8 ohms (fluctuating). Repair: Repaired harness, rerouted wiring to avoid moving parts. Fault resolved.
BYD DTC AI Analysis

B-pillar connector water ingress and oxidation

✓
Vehicle: Qin Pro (ICE) 2018 model, 58,000 km. Airbag warning light activated after driving through approximately 30 cm of water. DTC B17721B present; live data showed 4.2 ohms resistance. Removed right B-pillar interior trim. Found pre-tensioner connector (yellow) had visible water stains with blackened, oxidized copper pins. Cleaned connector with electronic cleaner, applied conductive grease. Resistance restored to 2.3 ohms. Cleared DTC; no recurrence.
BYD DTC AI Analysis

Bridge wire aging inside pretensioner housing

✓
Vehicle: Qin Pro DM 2018, 65,000 km. No accident history. Airbag warning light intermittent. Fault appears during cold starts and clears when warm. Measured pretensioner resistance at 8.5 ohms cold, dropping to 3.2 ohms after slight heating. Diagnosed as internal bridge wire oxidation in the pretensioner causing poor contact. Repair: Replaced right rear seatbelt pretensioner assembly (must be replaced with the seatbelt as a unit, not separately). Coding matched; fault resolved.
BYD DTC AI Analysis

Aftermarket carpet causing poor ground connection

✓
Vehicle: Qin Pro Petrol 2019, 21,000 km. Customer fitted a 360° aviation soft-wrap carpet, setting DTC B17721B. Found the right rear sill trim was removed during the install. The pretensioner ground point (on the right rear longitudinal rail inner panel) wasn't tightened properly, and the paint wasn't sanded, increasing contact resistance. Fix: Sanded the ground point to bare metal, tightened the bolt to spec (normally 8–10 N·m), cleared the code. No recurrence.
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.