AU
B171C

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

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 — Seal U
  • 1Left knee airbag igniter internal short circuit (inter-turn short circuit caused by damaged internal squib coil insulation)
  • 2Airbag wiring harness short to ground (wiring harness under the instrument panel chafes against the body metal bracket, damaging the insulation and causing a ground fault)
  • 3Short circuit between airbag connector terminals (water ingress and corrosion after wading, metal swarf bridging, or terminal deformation causing contact)
  • 4Improperly secured wiring harness after accident repairs (mounting bracket pinched the wiring harness during dashboard removal and installation, causing a short circuit)
  • 5ACU internal drive circuit fault (power transistor breakdown inside the control unit causes 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. Record the freeze frame data.
  • 2
    Visually inspect the lower left knee area of the dashboard for signs of impact, water ingress, or unauthorized modifications (such as installed metal pedals).
  • 3
    Disconnect the battery negative terminal and wait at least 3 minutes to fully discharge the ACU energy storage capacitor and prevent accidental airbag deployment.
  • 4
    Remove the lower left dashboard trim panel and locate the left knee airbag module (marked by a yellow connector). Disconnect the airbag connector and short the harness-side terminals (to prevent static discharge).
  • 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. A reading of <1Ω indicates an internal short circuit in the airbag module. Replace the airbag module.
  • 6
    Measure the resistance between each wiring harness pin and body ground: Standard is >1 MΩ (infinite). If continuity exists, inspect the wiring harness along the routing 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. 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 to verify the ACU no longer reports a fault.
  • 9
    Reinstall the actual airbag module, restore power, and clear the fault code. Perform an SRS system self-check (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 Seal U 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.