AU
B171C

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

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.

The B171C fault code on the Qin Plus indicates a medium-severity short circuit in the driver side knee airbag igniter circuit, with resistance detected too low. Common causes include an internal short in the knee airbag igniter or damaged under-dash airbag wiring shorting to ground. The airbag warning lamp may turn on and knee airbag protection may be disabled. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Internal short circuit in the left knee airbag igniter (damaged insulation on the internal squib coil causing an inter-turn short circuit)
  • 2Airbag wiring harness short to ground (wiring harness under instrument panel chafes against body metal bracket, causing insulation damage and 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 (wiring harness pinched in mounting bracket during instrument panel removal and installation, causing a short circuit)
  • 5ACU internal drive circuit fault (internal power transistor breakdown in the control unit causing 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 does not clear, and record the freeze frame data.
  • 2
    Visually inspect the lower left knee area of the dashboard for signs of impact, water ingress, or unauthorised modifications (such as added metal pedals).
  • 3
    Disconnect the battery negative terminal and wait at least 3 minutes for the ACU energy storage capacitor to discharge completely to prevent accidental airbag deployment.
  • 4
    Remove the lower left dashboard trim panel and locate the left knee airbag module (yellow connector). Disconnect the airbag connector and short the harness-side terminals (to prevent static electricity).
  • 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. If <1Ω, the airbag module has an internal short circuit. Replace the airbag module.
  • 6
    Measure the resistance from the two pins on the wiring harness side to body ground. The normal value is >1 MΩ (infinite). If continuity exists, inspect the wiring harness along the circuit for damage, focusing on the dashboard 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 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 the power supply, and clear the fault codes. Perform an SRS system self-test (turn the ignition switch to ON and verify 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 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.