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 8

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 8
  • 1Internal short circuit in the knee airbag module igniter: 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 chafing and short circuit: Vibration, friction, or crushing damages the insulation on the wiring harness from the ACU to the knee airbag (typically routed along the instrument panel frame or floor harness), causing a short circuit between the conductors or to the vehicle body.
  • 3Connector terminal short circuit: Deformed internal terminals, corrosion from water ingress, or metallic foreign objects inside the knee airbag connector (yellow, located under the left side of the dashboard) cause 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 the detected value is low).
  • 5Improper repair procedures: Measuring airbag terminals directly with a multimeter resistance setting during accident repairs (use a dedicated airbag resistance meter), or improper wiring harness routing causing damage from contact with sharp metal edges.
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). Read all fault codes using VDS or a dedicated diagnostic tool. Confirm if only B171D1A is present or if other airbag circuit faults accompany it.
  • 2
    Visual inspection and connector check: Inspect the knee airbag area under the dashboard for signs of impact, water stains, or removal/installation marks. Disconnect the knee airbag connector (yellow locking tab) and check the terminals for corrosion, deformation, or foreign objects.
  • 3
    Harness continuity measurement: Set a digital multimeter to the resistance range (do not measure with the airbag connected) and measure the harness between the ACU and the knee airbag connector: 1) resistance between the positive and negative wires (should be >1 MΩ, open circuit); 2) resistance from each wire to ground (should be >1 MΩ); 3) loop resistance with the harness disconnected from the airbag module (normal is 2-3 Ω; if <1.5 Ω, 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. The standard value is 2.0±0.3Ω. If the measured value 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 harness; if the connector is faulty, clean or replace the connector; if the airbag module is faulty, replace 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 12V battery negative terminal, use the diagnostic tool to clear the fault code, execute "SRS System Self-Learning" or "Configuration Reset", and perform the "Airbag System Function Check" (simulate a crash signal test to confirm DTC 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 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.