AU
B171D-00

DTC B171D-00 indicates the resistance of the driver-side left knee airbag ignition circuit is below the normal threshold set by the SRS control unit (typically below 1 — Seal U

Safety System

DTC B171D-00 indicates the resistance of the driver-side left knee airbag ignition circuit is below the normal threshold set by the SRS control unit (typically below 1.5Ω; the standard value is 1.6-2.4Ω).

This fault indicates a short circuit in the knee airbag ignition circuit.

Potential causes include an internal short in the airbag igniter coil, damaged wiring insulation causing a short between positive and negative circuits, or a short between connector terminals.

The SRS system identifies the airbag circuit as abnormal.

The knee airbag may fail to deploy during a collision or, in extreme cases, deploy unintentionally.

Consequently, the system illuminates the airbag warning light and logs this fault code.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal short circuit in the left knee airbag igniter: A manufacturing defect or prolonged high-temperature exposure damages the inter-turn insulation of the airbag module igniter coil, dropping resistance below 1Ω.
  • 2Wiring harness chafing and short circuit: Long-term vibration and friction against the metal bracket damage the knee airbag wiring insulation where it routes under the instrument panel near the steering column, causing direct contact between the positive and negative wires.
  • 3Connector water ingress and oxidation: During water crossings or interior cleaning, moisture enters the knee airbag connector (usually located low under the dashboard), decreasing insulation resistance between terminals or causing electrolytic corrosion.
  • 4SRS control unit detection circuit fault: Damage to the internal detection circuit component for the left knee airbag causes a false low resistance reading.
  • 5Improper repair procedures: Tools crush or retaining clips damage the knee airbag wiring harness during instrument panel removal and installation, carpet replacement, or pedal modification, causing a hidden short circuit.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS system capacitors and prevent accidental airbag deployment.
  • 2
    Fault code confirmation: Use the BYD VDS diagnostic tool to read fault codes. Confirm whether B171D-00 appears alone or with other SRS fault codes. Check the real-time knee airbag resistance value in the data stream.
  • 3
    Visual inspection: Remove the driver-side lower trim panel. Check the knee airbag connector (usually a yellow plug) for signs of water ingress, corrosion, or looseness. Check the wiring harness near the steering column for wear.
  • 4
    Resistance measurement: Disconnect the knee airbag connector and use a multimeter to measure the resistance on the airbag side (standard: 1.6-2.4 Ω). If the resistance is below 1.0 Ω, replace the airbag module. Measure the insulation resistance on the wiring harness side to ground, to power, and between the terminals. The resistance must be greater than 10 MΩ.
  • 5
    Wiring harness repair: If the wiring harness shows damage, strip the insulation and check the extent of wire damage. Repair a single broken wire by soldering. Replace and rewrap the entire harness section for multiple short-circuited wires.
  • 6
    Component replacement: If diagnostics confirm an internal short circuit in the knee airbag, replace the left knee airbag module (during installation, verify airbag orientation and fully engage retaining clips). If the wiring harness is normal but the fault persists, consider replacing the SRS control unit.
  • 7
    System reset: Reconnect all connectors and connect the battery. Use the diagnostic tool to clear the fault code and perform the 'SRS system self-check'. Confirm B171D-00 does not recur and the instrument cluster airbag warning lamp turns off normally.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV450 knee airbag connector shorted due to water ingress

✓
A 2018 Qin EV450 arrived with the airbag warning light constantly illuminated. Using VDS, we retrieved DTC B171D-00 (left knee airbag resistance too low). We removed the lower trim panel under the driver's side dashboard and found obvious water staining inside the knee airbag connector (yellow plug), with green corrosion on the terminals. The owner confirmed the vehicle had previously driven through water that reached floor level. Using a multimeter, we measured the knee airbag resistance at only 0.3Ω (severe short circuit). We cleaned and dried the connector terminals; the airbag unit resistance returned to the normal 2.1Ω. After reconnecting and clearing the DTC, the fault cleared. We recommend checking whether the AC drain hose is blocked, as this could cause water to leak onto the floor.
Original source ↗
BYD DTC AI Analysis

BYD Qin 100: Wiring harness chafed and shorted after accident repair

✓
A 2017 BYD Qin 100 came in with an illuminated airbag warning light after the dealership replaced the instrument panel assembly. The diagnostic tool showed DTC B171D-00. During installation of the new panel, the edge of the metal bracket pinched the left knee airbag wiring harness, damaging the insulation and shorting two internal wires. Resistance at the short measured 0.5Ω. The technician cut out the damaged section, re-soldered the joints and applied double-layer insulation, rerouted the harness away from the sharp bracket edges, and wrapped the harness with fleece tape for extra protection. After clearing the code, the system returned to normal.
BYD DTC AI Analysis

BYD Qin EV300 knee airbag internal squib short circuit

✓
A 2017 BYD Qin EV300 had an intermittent SRS light. The light sometimes turned off by itself after driving a distance. The fault occurred more often during cold starts. Retrieved DTC B171D-00. Live data showed the left knee airbag resistance fluctuating between 0.8–1.2 Ω, below normal. Disconnected the airbag connector and measured the airbag directly: 0.9 Ω and unstable. This indicated an intermittent short in the internal igniter coil. Replaced the left knee airbag module (Part No. SC-5820100). The new airbag measured 2.0 Ω, the standard value. Performed SRS configuration after installation. That completely resolved the fault.
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.