AU
B171C

DTC B171C indicates the Airbag Control Unit (ACU) detects 0 ohms in the Driver Side Knee Airbag igniter circuit — Atto 3

Safety System

DTC B171C indicates the Airbag Control Unit (ACU) detects 0 ohms in the Driver Side Knee Airbag igniter circuit.

This points to a short to ground in the circuit or an internal short in the igniter.

In the BYD SRS system, normal airbag igniter resistance is 2.0-5.0Ω.

When the ACU detects circuit resistance below the threshold (typically <0.8Ω), it registers a short circuit fault, illuminates the airbag warning lamp, and disables the airbag to prevent accidental deployment.

During a frontal collision, the driver loses knee airbag protection, risking severe lower limb injury.

This fault does not cause unintended airbag deployment.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Internal short circuit in the left knee airbag igniter (damaged internal squib coil insulation causing an inter-turn short circuit)
  • 2Airbag wiring harness short to ground (wiring harness under dashboard chafes against body metal bracket, causing wear and grounding)
  • 3Short circuit between airbag connector terminals (water ingress and corrosion after wading, metal shavings bridging, or terminal deformation causing contact)
  • 4Improperly secured wiring harness after accident repairs (wiring harness pinched in a mounting bracket during dashboard removal and installation, causing a short circuit)
  • 5ACU internal drive circuit fault (control unit internal power transistor breakdown causing false detection)
What To Do
  • 1
    Use the BYD VDS2000 or a dedicated diagnostic tool to read the DTC, confirm B171C is a current fault (Active) that will not clear, and record the freeze frame data.
  • 2
    Visually inspect the lower left dashboard knee area for signs of impact, water ingress, or unauthorized modifications (such as aftermarket metal pedals).
  • 3
    Disconnect the battery negative terminal and wait at least 3 minutes to ensure the ACU energy storage capacitor discharges completely, preventing accidental airbag deployment.
  • 4
    Remove the lower left dashboard trim panel and locate the left knee airbag module (yellow connector). Disconnect the airbag connector and short the harness-side terminals (to prevent static electricity).
  • 5
    Use a high-precision digital multimeter (0.1Ω resolution) to measure the resistance between the two pins on the airbag side. Normal resistance is 2-5Ω. If the resistance is <1Ω, the airbag module has an internal short circuit. Replace the airbag module.
  • 6
    Measure the resistance between the two pins on the wiring harness side and body ground. Normal value: >1 MΩ (infinite). If continuity exists, trace the circuit to inspect the wiring harness for damage, focusing on the instrument panel frame pass-through holes and retaining clips.
  • 7
    Check the connector terminals for corrosion, burn marks, or mechanical deformation. Clean with electrical contact cleaner and replace the wiring harness connector if necessary.
  • 8
    After completing the repair, connect an airbag simulator (2.7Ω resistor) to the circuit in place of the actual airbag and verify the ACU no longer reports a fault.
  • 9
    Refit the actual airbag module, restore power supply, and clear the fault codes. Perform an SRS system self-test (turn the ignition switch to ON and observe the warning light turns off after 6 seconds). Perform a road test to confirm.
Real-World Cases
BYD DTC AI Analysis

Knee airbag short circuit repair after water damage

✓
A 2019 BYD Yuan EV had a steady airbag warning light on the dash after driving through heavy rain and flood water. Pulled DTC B171C (left knee airbag resistance 0). Inspection found the water depth had exceeded the door sills, with standing water in the left front footwell. Removed the left knee airbag connector – found water traces and verdigris corrosion inside the plug. Cleaned the connector terminals with anhydrous alcohol and blew them dry. The wiring harness tested normal, but the airbag module was internally shorted (0.3 Ω). Replaced the left knee airbag module, repaired the harness waterproof seal, and resolved the fault.
BYD DTC AI Analysis

Wiring harness pinched after accident repair, causing short circuit

✓
2021 BYD Tang developed a B171C fault code one week after front collision repairs. Service history showed the instrument panel had been removed and refitted to replace the A/C evaporator. Inspection found the instrument panel mounting bracket had pinched the left knee airbag wiring harness, damaging the insulation and causing the copper wires to short to ground. Rerouted the wiring, installed protective corrugated tubing, and repaired the damaged wire insulation. Fault resolved.
BYD DTC AI Analysis

Airbag module internal igniter short circuit

✓
2019 Song MAX: Airbag warning light illuminated without warning during normal driving. Scan tool showed current DTC B171C. After disconnecting the airbag connector, harness-side resistance to ground measured normal (infinite) and supply voltage measured normal. Airbag module side resistance measured 0.8 Ω (internal short). Replaced the left knee airbag assembly; system self-test passed. Cause: internal manufacturing defect in airbag igniter, or coil insulation failure from long-term vibration.
BYD DTC AI Analysis

Connector short circuit due to metallic debris bridging terminals

✓
2020 BYD Qin petrol. Owner dropped a metal fastener beneath the dashboard when installing floor mats. B171C fault appeared a few days later. Inspection revealed a metal washer shorting across two terminals in the left knee airbag connector. Cleared the debris and restored normal system operation. Advised the owner to keep metal objects away from airbag assemblies.
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.