AU
B17721B

DTC B17721B indicates the airbag system (SRS) detects the right rear seat belt pretensioner circuit resistance exceeds the normal upper threshold — Atto 8

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.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Poor connector contact: Moisture, oxidation, or vibration increases pin contact resistance at the pretensioner connector (usually marked R2R or SA3) on the right B-pillar or under the seat. This is the most common cause, frequently occurring after wading or car washes.
  • 2Harness damage causing high resistance: Crushing the harness during rear seat removal/installation, carpet modification, or child seat installation partially breaks internal copper strands without fully severing them, creating a high-resistance point.
  • 3Pretensioner internal fault: Oxidation or breakage of the squib bridge wire, or moisture ingress into the pyrotechnic charge, causes resistance to drift higher 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 for the corresponding channel inside the airbag control module triggers a false fault (relatively rare; consider only after ruling out wiring faults).
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 from the freeze frame data (usually displays as 4.5-10 ohms or Open).
  • 3
    Visual inspection: Remove the right rear seat. Check 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. Check 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 the resistance directly across the pretensioner terminals. Normal value: 2.0 ± 0.5 ohms. If the 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; it should be <1 ohm. Measure the insulation resistance to ground; it should be >10M ohms. Inspect the wiring harness pass-through at the B-pillar for internal breaks.
  • 6
    Repair verification: After repairing or replacing the faulty component, reconnect the battery, clear the fault codes, and perform an SRS system self-test (turn the ignition switch to the ON position and observe if the airbag warning light turns off after 6 seconds). If necessary, perform a simulated crash test (use a dedicated substitute resistor 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 8 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.