AU
B171C-00

DTC B171C-00 indicates the driver's knee airbag circuit resistance is 0 Ω, representing a hard short — Qin Plus

Safety System

DTC B171C-00 indicates the driver's knee airbag circuit resistance is 0 Ω, representing a hard short.

In the SRS (Supplemental Restraint System), normal airbag squib resistance is 2.0-3.0 Ω.

A 0 Ω reading indicates a line short to ground between the SRS ECU and the knee airbag, an internal short in the airbag squib, or a short caused by abnormal connector terminal contact.

This fault triggers the SRS fail-safe mode.

The airbag may fail to deploy during a collision, or in extreme cases, the wiring fault could cause unintended deployment (a low-probability theoretical risk).

Because the knee airbag sits in front of the driver's knees (beneath the dashboard), water ingress, physical impacts, or aftermarket modifications easily affect this area.

Consequently, it is one of the SRS components most prone to short circuits.

3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Loose knee airbag connector (yellow plug), backed-out terminals, or water ingress corrosion causing a short circuit between terminals or a short to ground. This commonly occurs after vehicle wading or improper cabin cleaning.
  • 2Internal short circuit in the knee airbag igniter (generator). This internal component fault typically accompanies airbag assembly aging or previous external impact (such as a forceful knee strike).
  • 3Damaged wiring harness insulation causing a short to ground, especially under the steering column or near the instrument panel frame, where vibration and friction wear through the harness and cause contact with the vehicle body metal.
  • 4SRS ECU internal detection circuit fault causing incorrect circuit resistance readings. This rare condition typically triggers multiple unrelated airbag fault codes simultaneously.
  • 5Improper vehicle modifications (such as installing a dashcam or footwell lights) where screws pierce the wiring harness or metal brackets pinch the wiring harness, causing a short circuit.
What To Do
  • 1
    Safety preparation: Turn the vehicle OFF, disconnect the 12V battery negative terminal, and wait at least 3 minutes to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Locate component: Remove the driver-side lower instrument panel trim (knee bolster). Locate the left knee airbag module (usually below the steering column) and the yellow 2-pin connector.
  • 3
    Initial inspection: Check the connector for looseness, backed-out terminals, signs of water ingress, or corrosion. Check the wiring harness for wear or crush marks. Pay special attention to interference points with the steering column and instrument panel frame.
  • 4
    Resistance measurement: Disconnect the knee airbag connector and use a multimeter to measure the resistance on the airbag side (should be 2-3Ω). If 0Ω, replace the airbag assembly. Measure the resistance to ground on the wiring harness side (ECU side). If 0Ω, the wiring harness is shorted to ground.
  • 5
    Wiring harness repair: If the wiring harness is damaged, wrap it with insulating tape or replace the wiring harness. Securely fasten the wiring harness, prevent interference with metal components, and maintain clearance from sharp edges.
  • 6
    Component replacement: If replacing the knee airbag, use genuine parts (match the part number). During installation, fully engage the connector locking tab and listen for a 'click' to confirm the lock.
  • 7
    System reset: Reconnect the battery, set the vehicle to ON, use the diagnostic tool to clear the fault code, perform 'SRS System Self-check' or 'Configuration and Settings', and confirm B171C-00 no longer appears and the airbag warning light turns off.
  • 8
    Road test verification: Perform a bumpy road test to confirm no intermittent faults exist, and verify the SRS system has no current DTCs.
Real-World Cases
BYD DTC AI Analysis

Loose connector caused a short circuit.

✓
Symptom: The airbag warning light on the dashboard stayed on constantly. A diagnostic scan retrieved DTC B171C-00. The vehicle had no accident history and the fault appeared suddenly. Diagnosis: Per the workshop manual, B171C-00 indicates zero resistance in the driver's knee airbag circuit. The technician removed the driver-side knee bolster and inspected the knee airbag connector, finding the yellow airbag plug loose with poor pin contact causing the abnormal resistance reading. He measured the airbag harness with a multimeter; the resistance read 0Ω (normal 2-3Ω). Fix: Disconnected the negative battery terminal and waited 3 minutes. Reseated the knee airbag connector firmly, ensuring the locking tab clicked into place. Cleared the DTC and powered on the ignition for self-test; the fault code cleared and the warning light went out.
Original source ↗
BYD DTC AI Analysis

Internal short circuit in knee airbag.

✓
Symptoms: During routine service, a technician found a stored DTC B171C-00 in the airbag system that could not be cleared. The vehicle had no collision history but had undergone interior trim modifications. Diagnosis: The technician first checked the wiring harness connectors and found no faults; disconnected the airbag connector and measured resistance directly across the airbag unit pins, finding 0Ω (normal is approximately 2–3Ω). This indicated an internal squib short circuit in the knee airbag or an internal fault within the airbag module. Resolution: Replaced the left knee airbag assembly (part number to be confirmed according to specific model). Reconnected the component, used a scan tool to perform 'Configuration and Setup' programming, cleared the DTCs, and the system returned to normal.
Original source ↗
BYD DTC AI Analysis

Wiring harness chafed through, shorting to ground

✓
Symptoms: The airbag warning lamp illuminates intermittently when driving on rough roads. DTC B171C-00 (current fault) present. The fault is intermittent and vibration-related. Diagnosis: The technician inspected the knee airbag connector and found it normal. When tracing the wiring harness to the SRS ECU, the technician found the left knee airbag harness chafing against a metal bracket near the steering column. The worn insulation caused a short to earth. Measured resistance to earth: 0Ω. Solution: Repaired the damaged wiring (wrapped with insulation tape or replaced that section), rerouted and secured the harness to prevent interference with metal components. Cleared the DTC and road tested to confirm the fault was resolved.
Original source ↗
Other Qin Plus 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.