AU
B17661B

DTC B17661B indicates the airbag control unit (SRS ECU) failed to detect a valid electrical connection or the physical presence of the right front side curtain airbag during the system self-check — Atto 3

Safety System

DTC B17661B indicates the airbag control unit (SRS ECU) failed to detect a valid electrical connection or the physical presence of the right front side curtain airbag during the system self-check.

This "module missing/configuration error" fault signifies a complete communication loss between the ECU and the airbag module, rather than a simple igniter resistance anomaly.

Root causes typically include missing airbag module power supply (12V constant or IG power), poor ground circuit, open or short circuits in the LIN/CAN communication lines, internal airbag module MCU fault, loose connections, backed-out pins, or oxidation at the wiring harness connector, or incorrect vehicle configuration data (e.g., failing to perform online coding after replacing the airbag module).

This fault prevents the right front side curtain airbag from deploying during a collision, severely compromising occupant side impact protection.

In certain repair scenarios and specific model years, this DTC also indicates a communication loss between the battery management system (BMS) and the vehicle control unit (VCU).

Differentiate the diagnosis based on specific symptoms (airbag warning indicator illuminated vs. powertrain fault) and accompanying DTCs.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Right front curtain airbag module wiring harness connector (located above the right A-pillar or along the headliner edge) is loose, has backed-out terminals, oxidation, corrosion, or poor contact, interrupting power supply or communication.
  • 2Open circuit, short to power, or short to ground in the communication line (LIN bus or CAN bus) between the airbag module and the SRS ECU, especially at the A-pillar and headliner wiring harness bends where pinching or chafing causes intermittent faults.
  • 3Internal circuit fault in the right front curtain airbag module (damaged power management chip, failed communication chip) or open igniter circuit causing no module response.
  • 4SRS ECU configuration data error, such as failing to perform online configuration after replacing the airbag module, a vehicle VIN code mismatch with the airbag system, or an outdated ECU software version causing identification failure.
  • 5Low-voltage power supply system fault, such as low battery voltage (below 9V) or a loose ground point for the SRS ECU or airbag module, causing a communication timeout during self-check.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000/3000) to read the complete fault code stream and freeze frame data. Record the vehicle status when the fault occurred (vehicle speed, voltage, temperature). Check for accompanying U-class communication fault codes or related B1766 series fault codes.
  • 2
    Turn off the ignition switch and wait at least 90 seconds (for capacitor discharge). Remove the right A-pillar trim panel. Visually inspect the right front curtain airbag module connector (usually yellow) for looseness, backed-out terminals, oxidation, or corrosion. If necessary, clean the connector with electrical contact cleaner, apply conductive grease, and reconnect it.
  • 3
    Disconnect the airbag module connector. Measure the voltage at the connector-side power supply terminal (should be 12V battery voltage) and the resistance at the ground terminal (should be less than 1Ω). Measure the LIN or CAN line voltage (LIN: approx. 11V or fluctuating between 0-12V; CAN-H: approx. 2.6V; CAN-L: approx. 2.4V). Check the wiring harness for continuity and insulation to ground/power.
  • 4
    Use a multimeter to measure the airbag module squib circuit resistance (standard value is typically 2.0-3.0Ω, varying slightly by model). Check for an open circuit (infinite resistance) or a short circuit (close to 0Ω). Use low-current mode (<1mA) during measurement to prevent accidental deployment.
  • 5
    If all circuit measurements are normal, attempt to update the SRS ECU software or perform online configuration (coding matching). Use the diagnostic tool to perform 'System Configuration' and 'Component Protection Removal'. If the fault persists, replace the right front curtain airbag module and perform coding configuration and safety configuration for the new module.
  • 6
    Clear the fault code and cycle the key (OFF-ACC-ON) three or more times. Confirm the SRS warning light turns off after the self-check (illuminates for 6 seconds, then turns off). Use the diagnostic tool to perform the 'System Configuration Verification' test. If necessary, perform an airbag circuit simulation test (requires special equipment) and confirm complete fault resolution.
Real-World Cases
BYD DTC AI Analysis

Song Pro DM-i power limitation - Loose VCU connector causes B17661B

✓
Symptoms: Instrument cluster displayed "Check Powertrain". Vehicle would not move; no response to gear selection. Scan tool retrieved DTC B17661B (communication fault between battery management system and vehicle control unit). Diagnostic process: Checked BMS power supply (12V) and CAN bus voltages (CAN-H 2.6V, CAN-L 2.4V), both normal. Further inspection found loose VCU connector with internal oxidation. Solution: Cleaned and reconnected VCU connector, cleared fault codes. Road test normal.
Original source ↗
BYD DTC AI Analysis

Qin PLUS EV sudden power loss - BMS power module damaged

✓
Symptoms: Sudden loss of power while driving. The instrument cluster displayed "Power System Fault" with DTCs B17661B and U012187 (lost communication with BMS). Diagnosis: VDS2000 showed no communication with the BMS. Fuse No. 30 (constant power) in the front compartment fuse box had blown. Replaced the fuse, but the fault persisted; the BMS internal power module was damaged. Solution: Replaced the battery management system (BMS) assembly and completed coding and matching. Repair cost: approximately 2800 yuan.
Original source ↗
BYD DTC AI Analysis

Han EV Won't Start After Standing Overnight — DC-DC Wake Signal Line Open Circuit

✓
Symptoms: Vehicle failed to start after sitting overnight. Dashboard blank, start button unresponsive. Started with external jump-start but fault recurred. Historical DTCs included B17661B. Diagnosis: Low voltage battery measured at only 10.2V. DC-DC converter operating intermittently. Open circuit found in wake-up signal line between DC-DC and BMS inside left front fender. Solution: Repaired wake-up signal line and replaced degraded high-voltage harness connector.
Original source ↗
BYD DTC AI Analysis

Tang DM fast charging interrupted – excessive battery cell voltage differential triggered protection

✓
Symptoms: Fast charging suddenly cut out. Dashboard showed charging fault; DTCs B17661B and B176200 stored. Diagnosis: Fast charging port and wiring harness checked normal. CAN bus waveforms normal. BMS live data showed 180mV delta between highest and lowest cell voltages (threshold: <100mV). BMS triggered protection to disable charging. BMS-VCU communication fault stored DTC B17661B. Resolution: Performed cell balancing on the HV battery, replaced the module with faulty voltage sampling, and upgraded BMS control software.
Original source ↗
BYD DTC AI Analysis

Yuan Pro Intermittent Powertrain Fault – Poor Contact at BMS Power Supply Connector

✓
Symptoms: At 3,000 km, intermittent "Check power system" warning on the dash; light cleared after a few seconds. No cause found after multiple checks. Diagnosis: Freeze frame data when DTC B17661B set showed BMS supply voltage dropping instantly from 12.4V to 8.7V. Found loose connection at BMS supply connector X111 (beneath centre console) causing intermittent open circuit from driving vibration. Solution: Replaced BMS supply wiring harness assembly and added retaining clips to prevent loosening.
Original source ↗
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.