AU
B171D

DTC B171D indicates the driver-side knee airbag igniter circuit resistance falls below the lower threshold calibrated by the SRS control unit (ACU), typically under 2 — Qin Plus

Safety System

DTC B171D indicates the driver-side knee airbag igniter circuit resistance falls below the lower threshold calibrated by the SRS control unit (ACU), typically under 2.0 Ω.

In the BYD SRS system, the knee airbag serves as a critical restraint system component, with a standard igniter resistance range of 2.0–3.0 Ω.

A low resistance value indicates a short circuit risk.

Potential causes include an internal short in the igniter assembly, a short between wiring harness connector pins, or a wiring harness short to ground or power.

This fault causes the ACU to classify the airbag circuit as unsafe.

During a collision, the knee airbag may fail to deploy, or in extreme cases, deploy unintentionally.

Consequently, the SRS system illuminates the airbag fault warning lamp and may disable the entire airbag system.

3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Knee airbag igniter (squib) internal coil short circuit: Manufacturing defects or prolonged vibration cause an inter-turn short circuit in the airbag igniter's internal coil, reducing resistance.
  • 2Airbag wiring harness connector short circuit: The knee airbag connector (usually a yellow plug) located on the lower left side beneath the dashboard short-circuited due to water ingress and corrosion from wading or improper vehicle cleaning, or foreign matter bridging the terminals.
  • 3SRS wiring harness damage and short circuit: Long-term friction or pinching damages the harness insulation where it passes through the dashboard frame or near the steering column, causing a short circuit to body ground or the 12V power supply line.
  • 4Short circuit caused by improper operation: Screws pierce or crush the SRS wiring harness when installing a dashcam, modifying the audio system, or fitting an underbody shield, causing internal conductors to make contact.
  • 5ACU internal sampling circuit fault: A faulty internal A/D converter or sampling resistor in the airbag control unit causes a false low resistance reading in the knee airbag circuit.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes (to completely discharge the SRS capacitor). Read and record all fault codes. Check for accompanying fault codes such as B171C (resistance too high) and B171E (short to ground).
  • 2
    Visual inspection: Remove the driver-side knee panel. Inspect the knee airbag connector for looseness, water ingress, pin corrosion, or foreign matter. Inspect the wiring harness near the steering column for signs of wear or crushing.
  • 3
    Resistance measurement (airbag side): Disconnect the knee airbag connector. Measure the resistance directly between the two airbag inflator terminals using a digital multimeter (resistance setting). Standard value: 2.0-3.0Ω. If the resistance is <1.5Ω or close to 0Ω, replace the knee airbag assembly.
  • 4
    Harness continuity check: With the airbag connector disconnected, measure the resistance between the two pins on the harness side (ACU side). The reading must show an open circuit (OL). Any measured resistance indicates a short circuit between the harness wires. Measure the resistance from each pin to ground and to the 12V power supply to confirm no short circuit exists.
  • 5
    ACU and wiring harness inspection: If the wiring harness is normal, check if the ACU connector pins are backed out or corroded. Use an oscilloscope or dedicated diagnostic tool to read the ACU real-time data stream and verify the knee airbag resistance value is within the normal range.
  • 6
    Fault repair and verification: After repairing or replacing the faulty wiring harness/airbag, reconnect all connectors and the battery. Clear the fault code using a diagnostic tool and perform the SRS system self-check (usually requires 3 ignition cycles). Verify B171D does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Knee airbag connector corroded on water-wading vehicle, causing a short circuit

✓
A 2019 BYD Song MAX owner reported the airbag warning light stayed on. Diagnostic tool read DTC B171D (left knee airbag resistance too low). Inspection found water had accumulated below the A-pillar due to a blocked sunroof drain tube. The knee airbag connector (located under the left side of the dashboard) had internal water staining and green corrosion. After removing the connector and cleaning the oxides, the wiring harness side measured normal, but the airbag side read only 0.8 Ω (internally shorted). Replaced the left knee airbag assembly, sealed the connector. Fault resolved.
BYD DTC AI Analysis

Aftermarket underbody shield crushed SRS wiring harness

✓
A 2021 BYD Tang DM stored DTC B171D and the airbag warning light came on after installing an engine undertray. Inspection found the mounting screws were too long and had pierced the SRS harness sleeve beneath the dashboard, crushing two wires in the knee airbag circuit and causing a short. Repaired the harness by cutting out the damaged section, soldering the wires, and insulating with heat-shrink tubing. Replaced the undertray screws with shorter ones. Fault cleared.
BYD DTC AI Analysis

ACU internal sampling resistor drift caused a false alarm

✓
A 2020 BYD Qin petrol variant repeatedly set DTC B171D. The code would reappear after clearing and subsequent driving. Inspected the knee airbag and wiring harness; both were normal. Measured resistance: approximately 2.3 ohms (within specification). Suspected an internal ACU fault. Swapped the ACU into an identical vehicle for cross-verification; the fault transferred. Replaced the airbag control unit (ACU), recoded and matched it, completely eliminating the fault. Root cause: internal resistance drift in the ACU sampling circuit, causing false detection.
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.