AU
B171D

DTC B171D indicates the driver-side knee airbag igniter circuit resistance falls below the lower threshold calibrated by the SRS control unit (ACU), typically under 2 — Atto 3

Safety System

DTC B171D indicates the driver-side knee airbag igniter circuit resistance falls below the lower threshold calibrated by the SRS control unit (ACU), typically under 2.0 Ω.

In the BYD SRS system, the knee airbag serves as a critical restraint system component, with a standard igniter resistance range of 2.0–3.0 Ω.

A low resistance value indicates a short circuit risk.

Potential causes include an internal short in the igniter assembly, a short between wiring harness connector pins, or a wiring harness short to ground or power.

This fault causes the ACU to classify the airbag circuit as unsafe.

During a collision, the knee airbag may fail to deploy, or in extreme cases, deploy unintentionally.

Consequently, the SRS system illuminates the airbag fault warning lamp and may disable the entire airbag system.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Knee airbag inflator (squib) internal coil short circuit: Manufacturing defects or prolonged vibration cause an inter-turn short circuit in the inflator internal coil, decreasing the resistance value.
  • 2Airbag wiring harness connector short circuit: The knee airbag connector under the left side of the dashboard (usually a yellow plug) shorts due to water ingress and corrosion from driving through water or improper vehicle cleaning, or from foreign matter bridging the terminals.
  • 3SRS wiring harness damage and short circuit: Long-term friction or compression where the harness passes through the dashboard frame or near the steering column damages the insulation, causing a short circuit to body ground or the 12V power wire.
  • 4Short circuit caused by improper operation: Screws pierce or crush the SRS wiring harness when installing a dash cam, modifying the audio system, or installing an underbody guard, causing internal conductors to contact.
  • 5ACU internal sampling circuit fault: A fault in the airbag control unit's internal A/D converter or sampling resistor causes the ACU to incorrectly detect the knee airbag circuit resistance as too low.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes (to allow the SRS capacitor to discharge completely). Read and record all fault codes. Check for accompanying related fault codes such as B171C (resistance too high) and B171E (short to ground).
  • 2
    Visual inspection: Remove the driver-side knee panel. Inspect the knee airbag connector for looseness, water ingress, pin corrosion, or foreign object intrusion. Inspect the wiring harness near the steering column for wear or crush marks.
  • 3
    Resistance measurement (airbag side): Disconnect the knee airbag connector. Use a digital multimeter (resistance range) to measure the resistance directly between the two airbag inflator terminals. Standard value: 2.0-3.0 Ω. If the resistance is <1.5 Ω or close to 0 Ω, replace the knee airbag assembly.
  • 4
    Harness continuity check: Keep the airbag connector disconnected. Measure the resistance between the two pins on the harness side (ACU side). The display must show an open circuit (OL). Any measured resistance indicates a short circuit between the wires. Measure the resistance from each pin to ground and to the 12V power supply to confirm no short circuit exists.
  • 5
    ACU and wiring harness inspection: If the wiring harness is normal, inspect the ACU connector pins for push-out or corrosion. Use an oscilloscope or dedicated diagnostic tool to read the ACU real-time data stream and verify the knee airbag resistance value is within the normal range.
  • 6
    Fault Repair and Verification: After repairing or replacing the faulty wiring harness or airbag, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform the SRS system self-check (usually requires cycling the ignition switch 3 times). Verify B171D does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Knee airbag connector corroded on water-wading vehicle, causing a short circuit

✓
A 2019 BYD Song MAX owner reported the airbag warning light stayed on. Diagnostic tool read DTC B171D (left knee airbag resistance too low). Inspection found water had accumulated below the A-pillar due to a blocked sunroof drain tube. The knee airbag connector (located under the left side of the dashboard) had internal water staining and green corrosion. After removing the connector and cleaning the oxides, the wiring harness side measured normal, but the airbag side read only 0.8 Ω (internally shorted). Replaced the left knee airbag assembly, sealed the connector. Fault resolved.
BYD DTC AI Analysis

Aftermarket underbody shield crushed SRS wiring harness

✓
A 2021 BYD Tang DM stored DTC B171D and the airbag warning light came on after installing an engine undertray. Inspection found the mounting screws were too long and had pierced the SRS harness sleeve beneath the dashboard, crushing two wires in the knee airbag circuit and causing a short. Repaired the harness by cutting out the damaged section, soldering the wires, and insulating with heat-shrink tubing. Replaced the undertray screws with shorter ones. Fault cleared.
BYD DTC AI Analysis

ACU internal sampling resistor drift caused a false alarm

✓
A 2020 BYD Qin petrol variant repeatedly set DTC B171D. The code would reappear after clearing and subsequent driving. Inspected the knee airbag and wiring harness; both were normal. Measured resistance: approximately 2.3 ohms (within specification). Suspected an internal ACU fault. Swapped the ACU into an identical vehicle for cross-verification; the fault transferred. Replaced the airbag control unit (ACU), recoded and matched it, completely eliminating the fault. Root cause: internal resistance drift in the ACU sampling circuit, causing false detection.
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.