AU
B171C1A

DTC B171C1A indicates the Supplemental Restraint System (SRS) detects a 0Ω resistance in the Driver Knee Airbag ignition circuit, representing a typical short circuit fault — Atto 8

Safety System

DTC B171C1A indicates the Supplemental Restraint System (SRS) detects a 0Ω resistance in the Driver Knee Airbag ignition circuit, representing a typical short circuit fault.

Normal airbag inflator resistance measures between 2.0 and 3.0Ω.

A 0Ω resistance indicates a Line Short to Ground in the ignition circuit.

Possible causes include an internal short circuit in the airbag module, damaged wiring harness insulation grounding to the chassis, or a short circuit between connector terminals.

This fault causes the SRS control unit to flag the left knee airbag as unavailable, preventing deployment during a collision.

The control unit also illuminates the airbag warning light continuously.

The knee airbag is a critical restraint system component that protects the driver's lower limbs from dashboard intrusion.

Consequently, this fault significantly reduces protection performance during a frontal collision.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Left knee airbag module igniter short circuit: Moisture, aging, or manufacturing defects cause a short circuit between the positive and negative terminals of the heating wire or bridge wire inside the airbag.
  • 2Wiring harness short to ground: Wear, crushing, or water ingress damages the insulation on the wiring harness under the driver's side dashboard (typically routed along the steering column or carpet), causing a short to the vehicle body metal.
  • 3Connector fault: The SRS-specific yellow connector (usually located under the dashboard or center tunnel) has bent pins, corrosion from water ingress, or shorted terminals, resulting in 0 resistance.
  • 4Improper post-accident repair: Vehicle suffered a collision and the deployed knee airbag was not replaced, or improper wiring harness repair (e.g., directly connecting exposed wire ends without insulation).
  • 5Diagnostic interference: improper operation when measuring the airbag circuit with a multimeter (measure 90 seconds after disconnecting the battery), or an internal diagnostic tool fault causing a false report.
What To Do
  • 1
    Safety preparation: Switch off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS backup power supply and prevent accidental airbag deployment.
  • 2
    Visual inspection: Remove the driver's side lower dashboard trim panel (knee bolster). Check the left knee airbag module for damage, water stains, or burn marks. Check the wiring harness for obvious wear, crushing, or corrosion.
  • 3
    Connector inspection: Disconnect the connector (yellow plug) between the SRS control unit and the left knee airbag. Inspect the terminals for bending, backing out, corrosion, or short circuits caused by metallic foreign matter. Clean the terminals with electrical contact cleaner if necessary.
  • 4
    Resistance check: Use a digital multimeter (select the correct resistance range) to measure the resistance between the airbag module terminals. Normal resistance is 2.0-3.0Ω. Measure the resistance to ground on the wiring harness side. The reading should be infinite (OL). If the reading is 0, the wiring harness has a short circuit.
  • 5
    Harness insulation test: Inspect the left knee airbag harness section by section along its routing (from the SRS ECU to the airbag). Focus on sections passing through the door sill, under the carpet, and near the steering column. Repair damaged insulation and rewrap the harness.
  • 6
    Module replacement: If testing confirms an internal short circuit in the airbag module (resistance between module terminals measures 0), replace the left knee airbag assembly. Verify normal resistance of the new part before installation.
  • 7
    System reset: Reconnect all connectors and the battery. Use the VDS2000/VDS3000 diagnostic tool to access the SRS system, execute the 'Configuration and Settings' function, clear the fault code, and perform 'Sensor Calibration'.
  • 8
    Function check: Read the data stream to check the 'left knee airbag resistance' value and confirm it displays 2.0-3.0Ω. Perform an SRS system self-check to confirm no fault codes are present and the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

Knee airbag short circuit repair case for water-damaged vehicle

✓
A 2019 BYD Qin EV had the airbag warning light stay on after wading through deep water from a heavy rainstorm. Stored DTC: B171C1A. Disassembly revealed water pooled under the driver-side carpet. The left knee airbag connector (near the pedal below the A-pillar) had water inside and corroded, shorting the terminals. Repair: removed the water, cleaned the connector with electronic cleaner and dried it with compressed air. Measured harness insulation to ground—normal. Reconnected the connector, cleared the DTC, and resistance value returned to 2.3Ω. Fault resolved. Advise checking weatherstrips and drain holes to prevent water ingress again.
BYD DTC AI Analysis

Uninsulated wiring harness after accident repair caused short circuit

✓
2020 E3: Replaced the front bumper and left knee airbag following a minor collision. A B171C1A fault appeared the day after delivery. Found the technician had failed to reposition the corrugated tube on the airbag wiring harness during installation, allowing the harness to chafe through on the sharp edge of the dashboard metal bracket and the copper wires to short to earth. Re-wrapped the damaged harness using electrical tape and corrugated tube for double protection, rerouted the harness away from sharp edges, and measured resistance normal—fault resolved.
BYD DTC AI Analysis

Knee airbag module replacement due to internal short circuit

✓
2019 E2, no accident history. The SRS warning light suddenly illuminated with DTC B171C1A. Disconnecting the airbag connector and measuring the module’s own resistance gave 0.2Ω (normal 2–3Ω), indicating an internal squib short circuit. Further inspection traced the cause to heavy spraying of poor-quality interior cleaner beneath the dashboard. Liquid had penetrated the airbag module, causing internal corrosion. Solution: Replaced the left knee airbag assembly (part number matched to VIN). The new part measured 2.4Ω before installation. After fitting, cleared the fault codes; system self-test passed.
BYD DTC AI Analysis

Modified pedal damaged wiring harness, causing short circuit.

✓
A Qin EV owner fitted aftermarket metal pedals. When securing the left footrest bracket, the installer used self-tapping screws that were too long, piercing the knee airbag wiring harness underneath. This caused both positive and negative circuits to short to the screw (earth), triggering DTC B171C1A. During diagnosis, the fault code appeared intermittently; resistance values changed when wiggling the harness. Repair procedure: Remove the non-compliant pedal modification, replace the pierced section of wiring harness (or perform professional soldering and insulation), and reposition the harness to prevent interference with the pedal mechanism.
BYD DTC AI Analysis

Connector terminal bent and shorted

✓
A 2020 E3 set DTC B171C1A after a cabin air filter replacement at the dealership (which requires removing the glove box and lower trim panel). Suspected tool contact during the repair bent the terminals in the knee airbag connector on the left side of the centre console. Inspection found Pin 2 (power) and Pin 3 (earth) contacting each other due to terminal bending. Repair: Extracted the bent terminal using a dedicated removal tool, straightened it with needle-nose pliers, reinserted it ensuring proper clearance between terminals, and retested resistance – normal.
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.