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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 — Qin Plus

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.

The B171C1A fault code on the Qin Plus indicates a medium-severity short circuit in the driver knee airbag ignition circuit, causing the SRS to disable that airbag and turn on the warning light. Common causes include an internal short in the knee airbag module or damaged under-dash wiring shorting to ground. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Left knee airbag module internal igniter short circuit: Moisture, aging, or manufacturing defects cause the internal heating wire or bridge wire to short across the positive and negative terminals.
  • 2Harness short to ground: Wear, crushing, or water ingress damaged the insulation on the driver-side under-dash wiring harness (typically routed along the steering column or carpet), causing a short to the vehicle body metal.
  • 3Connector fault: Yellow SRS-specific 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: The vehicle suffered a collision and the deployed knee airbag was not replaced with a new part, or the wiring harness was improperly repaired (e.g., exposed wire ends directly connected without insulation).
  • 5Diagnostic interference: Measuring the airbag circuit with a multimeter without following specifications (measure 90 seconds after disconnecting the battery), or an internal diagnostic tool fault causing a false report.
What To Do
  • 1
    Safety preparation: Turn 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). Inspect the left knee airbag module for damage, water stains, or burn marks. Inspect 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 objects. Clean with electrical contact cleaner if necessary.
  • 4
    Resistance check: Use a digital multimeter (set to the correct resistance range) to measure the resistance between the airbag module terminals. The normal value is 2.0-3.0Ω. Measure the resistance to ground on the wiring harness side. The reading should be infinite (OL). A reading of 0 confirms a short circuit in the wiring harness.
  • 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 the harness passing along 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 on the new part before installation.
  • 7
    System reset: Reconnect all connectors and the battery. Use the VDS2000/VDS3000 diagnostic tool to enter the SRS system, execute the 'Configuration and Setting' function, clear the fault code, and perform 'Sensor Calibration'.
  • 8
    Function verification: 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 lamp 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 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus overview →
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.