AU
B171E

DTC B171E indicates the driver-side knee airbag (Left Knee Airbag Module) firing circuit resistance exceeds the ACU (Airbag Control Unit) calibrated threshold (typically >3 — Atto 8

Safety System

DTC B171E indicates the driver-side knee airbag (Left Knee Airbag Module) firing circuit resistance exceeds the ACU (Airbag Control Unit) calibrated threshold (typically >3.0Ω, standard value 2.0-3.0Ω).

Located under the left side of the dashboard, this airbag protects the driver's lower limbs from secondary impact during a frontal collision.

High resistance essentially indicates an open circuit or high-impedance state.

Causes include poor airbag connector contact, an open wiring harness inside the dashboard, an internal open circuit in the airbag inflator, or an ACU detection circuit fault.

This fault forces the SRS system into fail-safe mode.

It can prevent the knee airbag from deploying during a collision, continuously illuminate the instrument panel airbag warning light, and compromise the vehicle's passive safety system.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose or oxidized knee airbag wiring harness connector: Water ingress, damp conditions, or prolonged vibration causes terminal oxidation and increases contact resistance in the yellow dedicated airbag connector under the dashboard, creating a high-resistance state.
  • 2Dashboard internal wiring harness wear or open circuit: Dashboard removal and installation, cabin air filter replacement, or aftermarket equipment installation crushes, cuts, or causes a fatigue break in the knee airbag wiring harness near the steering column.
  • 3Knee airbag inflator fault: Aging, batch defects, or prolonged exposure to high temperatures cause an internal open circuit in the airbag inflator coil, resulting in infinite resistance.
  • 4Floor harness junction fault: Water ingress, corrosion, terminal pin back-out, or poor contact at the branch point from the main body harness to the knee airbag (usually below the driver-side A-pillar or at the floor harness connector).
  • 5ACU internal detection circuit fault: A damaged sampling resistor or detection circuit for the knee airbag inside the airbag control unit causes a false high resistance reading (verify by substitution).
What To Do
  • 1
    Safe power-down: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment and personal injury.
  • 2
    Initial inspection: Inspect the yellow dedicated connector for the left knee airbag under the dashboard (usually a 2-pin yellow housing). Verify the connector is securely seated, and check for broken locking tabs or obvious signs of water ingress, oxidation, burning, or terminal back-out.
  • 3
    Resistance measurement: Disconnect the knee airbag connector. Use a multimeter to measure the resistance on the airbag unit side. Normal resistance is 2.0-3.0Ω. A reading of infinity or >3.5Ω indicates an open circuit in the airbag unit or wiring.
  • 4
    Harness continuity test: Measure wiring harness continuity from the ACU (usually located under the middle of the center console or in the center floor tunnel) to the knee airbag connector. Resistance should be <1Ω. Check the wiring harness for a short to ground or a short to power.
  • 5
    Connector handling and repair: If oxidized, clean the terminals with electrical contact cleaner, apply special conductive grease, and reconnect the plug until you hear the locking 'click'. Replace the connector housing or repair backed-out terminals if necessary.
  • 6
    Replacement verification: If wiring harness continuity is normal and the connector is in good condition, but resistance remains high, replace the knee airbag assembly. (Note: The airbag is an explosive device. Handle it according to hazardous materials regulations. Verify the connector is completely disconnected during installation.)
  • 7
    System Reset and Function Test: Restore all connections, turn on the power, and clear the fault code using the dedicated diagnostic tool. Perform the 'SRS system self-check' or 'configuration setting'. Verify B171E does not return and the instrument panel warning light turns off. Finally, perform a road test to verify.
Real-World Cases
BYD DTC AI Analysis

Clock spring aging caused an open circuit, resulting in high resistance.

✓
Model: 2019 BYD Song Pro DM, 42,000 km. The airbag warning light remained on after startup. The scanner displayed DTC B171E (driver's side airbag circuit resistance too high, open circuit). Horn and steering wheel buttons functioned normally. Disconnected the battery negative terminal and removed the driver airbag. Measured 2.3 Ω directly at the airbag connector (normal), ruling out an airbag unit fault. Measured the wiring between the clock spring lower connector and the airbag connector; the yellow wire showed infinite resistance. Confirmed the clock spring internal ribbon cable had fractured from fatigue caused by repeated full-lock steering. Replaced the clock spring assembly with a genuine part (Part No.: SAS-3636100), calibrated the steering angle sensor, and cleared the fault codes. Road tested for 50 km with no fault recurrence.
Original source ↗
BYD DTC AI Analysis

Poor contact at airbag connector (intermittent abnormal resistance)

✓
2021 BYD Qin Plus DM-i, 18,000 km. Airbag light comes on intermittently, especially over bumpy roads or when turning the steering wheel. Sometimes it goes out after stopping and restarting. Scan tool shows code B171E (driver side airbag circuit resistance out of range, historic fault). Freeze frame data: fault occurred at 40 km/h with steering angle 380°. Removed and inspected the main airbag connector. Found slight oxidation on the internal terminals; the connector locking tab was not fully clicked into place. Measured connector contact resistance: 2.1 Ω static, jumping between 2–8 Ω when wiggling the steering wheel — confirmed poor contact. Cleaned the airbag connector terminals with electronic contact cleaner, applied a small amount of conductive grease, reconnected and made sure the locking tab clicked in. Secured the wiring harness with cable ties to prevent movement. Cleared fault codes and monitored for one month. No recurrence.
Original source ↗
BYD DTC AI Analysis

Wiring harness chafing caused short circuit and coding error after accident repair

✓
My 2020 BYD Han EV (35,000 km) was in a frontal collision. Replaced front bumper, left airbag sensor, and ACU. After repair, the instrument cluster displayed ‘Check Airbag System’. Pulled code B171E (driver airbag short to power/communication fault). Checked ACU power and ground — normal, CAN bus voltages normal. Measured driver airbag circuit and found a short to 12V. Tracing the harness revealed the airbag wiring had rubbed against the dash metal bracket during the repair, insulation worn through and shorting to body. Also found the replacement ACU was a used unit, coding didn’t match the VIN, causing a communication protocol error. Repaired the damaged wiring with heat shrink tubing, rerouted harness clear of the bracket. Used BYD ED400 dedicated diagnostic tool for online programming and matching on the ACU, wrote original vehicle configuration code, and executed SRS system configuration (Coding). Cleared fault codes, airbag function test normal.
Original source ↗
BYD DTC AI Analysis

Wiring harness under the seat corroded and shorted after wading through water

✓
2022 BYD Yuan Plus, 21,000 km. After driving through water about 30 cm deep, the airbag warning light stayed on, with intermittent horn failure. Diagnosis showed DTC B171E (driver-side airbag circuit low resistance, short-circuit tendency). Inspected the cabin floor: carpet damp, with signs of water pooling under the driver's seat. Removed and inspected the connector under the seat; found water ingress and corrosion. Green corrosion between pins caused a short circuit, and resistance measured only 0.8 Ω (too low). Also found moisture in the lower clock spring connector. Dried all wiring harness connectors with compressed air, cleaned pins with precision instrument cleaner, and replaced the 8-pin adapter connector under the seat (yellow airbag circuit connector). Applied waterproof silicone grease inside the connector and re-wrapped with waterproof tape. Removed water from the carpet, air-dried for 48 hours, then reassembled. Cleared fault codes; returned to normal.
Original source ↗
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.