AU
B171C

DTC B171C indicates the Airbag Control Unit (ACU) detects 0 ohms in the Driver Side Knee Airbag igniter circuit — Atto 8

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.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Left knee airbag igniter internal short circuit (damaged internal squib coil insulation causing an inter-turn short circuit)
  • 2Airbag wiring harness short to ground (wiring harness under dashboard chafed against body metal bracket, causing a short to ground)
  • 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 a mounting bracket during dashboard removal and installation, causing a short circuit)
  • 5ACU internal drive circuit fault (power transistor breakdown inside the control unit causes 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 will not clear. Record the freeze frame data.
  • 2
    Visually inspect the lower left knee area of the dashboard for signs of impact, water ingress, or unauthorized modifications (such as added metal pedals).
  • 3
    Disconnect the 12V battery negative terminal and wait at least 3 minutes for the ACU energy storage capacitor to discharge fully and 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 wiring harness-side terminals to prevent static discharge.
  • 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. A reading of <1Ω indicates an internal short circuit in the airbag module. Replace the airbag module.
  • 6
    Measure the resistance from the two wiring harness side pins to body ground: normal value is >1 MΩ (infinite). If continuity exists, inspect the wiring harness along the circuit for damage. Focus on the instrument panel frame pass-through holes and fixing clip locations.
  • 7
    Check 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 confirm the ACU no longer reports a fault.
  • 9
    Reinstall the actual airbag module, restore power, and clear the fault code. 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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.