AU
B171D1A

This DTC indicates the driver-side knee airbag module squib resistance falls below the SRS control unit (ACU) threshold (typically below 1 — Atto 3

Safety System

This DTC indicates the driver-side knee airbag module squib resistance falls below the SRS control unit (ACU) threshold (typically below 1.5Ω).

The knee airbag mounts on the left side of the dashboard below the steering wheel to protect the driver's knees and lower limbs during a frontal collision.

Low resistance typically indicates a short circuit in the squib circuit (short between wires, short to ground, or internal short within the airbag module), causing the ACU to detect abnormally low circuit resistance.

This fault forces the SRS system into degraded mode.

During a collision, the knee airbag may fail to deploy or deploy inadvertently.

Simultaneously, the instrument cluster airbag warning light remains illuminated, severely compromising passive safety functions.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Knee airbag module internal igniter short circuit: Moisture, manufacturing defects, or aging causes insulation failure between the positive and negative terminals of the igniter bridge wire inside the airbag module, dropping resistance below 1Ω.
  • 2Wiring harness abrasion/short circuit: Vibration, chafing, or pinching damages the insulation on the wiring harness from the ACU to the knee airbag (usually routed along the instrument panel crossmember or floor harness), causing a short circuit between core wires or a short to body ground.
  • 3Connector terminal short circuit: Deformed internal terminals, corrosion from water ingress, or metallic foreign objects inside the knee airbag connector (yellow, located below the left side of the dashboard) causing a short circuit between the positive and negative terminals.
  • 4SRS control unit internal sampling circuit fault: A fault in the ACU internal resistance sampling circuit or A/D converter causes a false low resistance reading (actual resistance is normal, but detected value is low).
  • 5Improper repair procedures: measuring airbag pins directly with a multimeter resistance setting during accident repairs (use a dedicated airbag resistance meter), or improper wiring harness routing causing contact with sharp metal edges and resulting in damage.
What To Do
  • 1
    Safety Preparation and Fault Confirmation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to discharge the SRS capacitor. Use VDS or a dedicated diagnostic tool to read all fault codes. Confirm if only B171D1A is present or accompanied by other airbag circuit faults.
  • 2
    Visual inspection and connector check: Check the knee airbag area under the dashboard for signs of impact, water stains, or removal/installation. Disconnect the knee airbag connector (yellow locking tab) and check the terminals for corrosion, deformation, or foreign matter.
  • 3
    Harness continuity measurement: Use a digital multimeter set to resistance (do not measure with the airbag connected) to measure the harness between the ACU and the knee airbag connector: 1) resistance between the positive and negative wires (should be >1MΩ, open circuit); 2) resistance from each wire to ground (should be >1MΩ); 3) loop resistance with the harness disconnected from the airbag module (normal is 2-3Ω; if <1.5Ω, the harness has a short circuit).
  • 4
    Airbag module resistance measurement: Use a dedicated airbag resistance meter (or connect a 2Ω protective resistor in series) to measure the resistance between the two terminals of the knee airbag module. Standard value: 2.0±0.3Ω. If the measurement is <1.5Ω or close to 0Ω, the airbag module has an internal short circuit. Replace the knee airbag assembly.
  • 5
    Repair or replace: If the wiring harness has a short circuit, repair the damaged insulation or replace the wiring harness; if the connector is faulty, clean or replace the connector; if the airbag module is faulty, replace it with an OEM knee airbag module (Note: Do not apply power to test the new module before installation).
  • 6
    System reset and verification: Connect all connectors, reconnect the battery negative terminal, use the diagnostic tool to clear the fault code, execute 'SRS system self-learning' or 'configuration reset', and finally perform the 'airbag system function check' (simulate a crash signal test to confirm B171D1A does not return and the warning light turns off).
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM Knee Airbag Module Internal Short Circuit Replacement Case

✓
The airbag warning light on the instrument panel stayed on when the vehicle arrived. VDS retrieved DTC B171D1A (left knee airbag resistance too low). After disconnecting the battery, I unplugged the knee airbag connector. The harness side resistance measured normally (open circuit), but resistance between the airbag module terminals measured only 0.3Ω, indicating an internal short in the airbag igniter. I replaced the left knee airbag module with a new unit (part number: SC-5820100B), cleared the DTCs and performed an SRS system self-test. This resolved the fault. Analysis revealed the airbag module's internal moisture-proof seal had failed, causing the bridge wire to corrode and short-circuit.
BYD DTC AI Analysis

Knee airbag connector corroded and short-circuited after a wading accident

✓
Water accumulated on the driver's side floor of a 2019 Qin PRO petrol due to a leaking front windscreen, and the airbag warning light subsequently illuminated. Diagnosis revealed DTC B171D1A. Inspection of the knee airbag connector (located inside the dashboard below the A-pillar) found obvious water staining and green copper corrosion inside the yellow connector, with insulation resistance between terminals measuring only 50 Ω. Cleaned the connector with electronic cleaner, dried it, and applied conductive grease; retesting showed normal resistance. Also repaired the windscreen seal, permanently fixing the fault.
BYD DTC AI Analysis

Wiring harness chafing caused an intermittent short circuit.

✓
Customer reported the airbag warning light occasionally illuminated, particularly when driving over rough roads. Scan tool showed historic fault code B171D1A. Inspected the knee airbag wiring harness routing and found the insulation had worn through at the dashboard frame mounting clip due to long-term vibration, exposing the copper core. When driving over bumps, the damaged section contacted the vehicle body metal, causing a short to earth and resulting in the ACU detecting a momentary resistance drop. Repaired the damaged harness using heat shrink tubing; rerouted and secured the harness to avoid contact with sharp edges. The fault has not reoccurred.
BYD DTC AI Analysis

Connector not locked after accident repair caused poor terminal contact

✓
After front collision repairs, the airbag warning light came on at delivery, storing DTC B171D1A. Inspection found the instrument panel had been removed and reinstalled during the work. The knee airbag connector was not fully locked after connection—the yellow secondary lock was not depressed—causing the terminals to contact and short together under vehicle vibration. Reconnected the connector, confirmed the audible click of the lock engaging, cleared the fault code, and the system returned to normal. Always verify the secondary lock on SRS connectors is fully engaged.
BYD DTC AI Analysis

SRS control unit software false alarm update case

✓
A 2018 Qin Pro DM frequently set DTC B171D1A, but the knee airbag and wiring harness resistance was normal at 2.1Ω. Replaced the airbag module and harness; the fault remained. BYD technical support confirmed a sampling algorithm defect in the ACU software for this production batch, causing false low-resistance readings at specific temperatures. Upgrading the ACU software from V1.2 to V1.4 permanently cleared the fault. This was a typical control unit software fault, not a hardware problem.
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.