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B171811

DTC B171811 indicates an abnormally low-resistance connection (typically <1 — Qin Plus

Safety System

DTC B171811 indicates an abnormally low-resistance connection (typically <1.5Ω) between the Driver Knee Airbag squib circuit and the vehicle chassis ground (GND).

In the SRS (Supplemental Restraint System), the squib circuit must maintain a high-impedance state (normally 2-3Ω).

The airbag control unit (ACU) logs a short to ground when it detects circuit resistance continuously below the threshold or abnormal current flow to ground.

This fault forces the ACU into an immediate fail-safe mode, disabling the knee airbag and potentially related airbag systems, and illuminates the instrument cluster airbag warning light.

Potential causes for this short to ground include contact between the internal squib bridge wire and the housing, damaged harness insulation shorting to the chassis, bent connector terminals touching metal components, or a breakdown of the internal ACU driver circuit.

This creates a dual safety risk of unintended airbag deployment (premature deployment) or failure to deploy during a collision.

Technicians must repair this hard fault immediately.

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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Internal short circuit in the knee airbag module: The internal ignition bridge wire contacts the metal housing, or moisture in the pyrotechnic element causes insulation failure, creating a path to ground.
  • 2Wiring harness physical damage: Long-term vibration, friction, or compression damages the insulation of the yellow SRS wiring harness under the dashboard, near the steering column, or in the pedal area, causing it to contact the vehicle body metal.
  • 3Connector fault: Water ingress, oxidation/corrosion, or terminal back-out at the knee airbag connector (usually located under the driver's side dashboard, marked in yellow) shorts the pin to body ground.
  • 4SRS control unit internal fault: ACU internal ignition drive transistor (IGBT) breakdown or filter capacitor short circuit, causing a continuous output signal to ground (less common but requires inspection).
  • 5Improper repair operations: During previous removal and installation of the instrument panel, steering column, or carpet, tools scraped the wiring harness, or excessively long fixing screws pierced the harness insulation.
What To Do
  • 1
    Safety preparation: Switch the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds (120 seconds for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation and freeze frame analysis: Use the BYD VDS2000 or X431 diagnostic tool to read the fault code. Confirm B171811 is a current fault (Active, not History). Record the resistance value (usually 0Ω or <1Ω) and the vehicle status from the freeze frame data.
  • 3
    Visual and physical inspection: Remove the driver-side lower trim panel (knee bolster). Inspect the knee airbag connector (yellow plug) for looseness, water ingress, green corrosion, or trapped foreign objects. Inspect the wiring harness for signs of chafing at the steering column retaining clip.
  • 4
    Circuit isolation measurement: Use a digital multimeter set to resistance (never measure with power applied). Disconnect the airbag module, then measure the igniter circuit-to-ground resistance on the wiring harness side. The normal reading is infinity (OL). If continuity exists, trace the wiring harness and inspect the pedal bracket and steering column metal parts for damaged insulation shorting to ground.
  • 5
    Component replacement verification: If the wiring harness is normal, replace the knee airbag module (Note: Never use a multimeter to measure airbag module terminal resistance or continuity, as the test current may deploy the airbag).
  • 6
    System configuration and reset: Reconnect all components and the battery, clear fault codes, execute the 'SRS System Configuration' or 'ECU Configuration' function (some models require writing the VIN and airbag coding), and perform a system self-check.
  • 7
    Final verification: Perform a road test and monitor the data stream. Verify the knee airbag resistance is within the 2-3Ω standard range and the insulation resistance to ground is >1MΩ. Verify the fault lamp is off and no intermittent faults exist.
Real-World Cases
BYD DTC AI Analysis

Broken clock spring caused circuit malfunction in BYD Song Pro DM

✓
The airbag warning light suddenly illuminated on the dashboard while driving a 2020 Song Pro DM and remained on after restarting. The technician used a VDS2000 to retrieve DTC B171811 (driver airbag first stage ignition circuit resistance too high or short to ground). The technician removed the steering wheel and measured infinite resistance at the airbag connector. Further disassembly revealed the flat cable inside the clock spring had fractured, causing the airbag circuit fault. The technician replaced the clock spring assembly with a genuine part (Part No.: SA-3636100), reassembled the steering wheel, performed airbag system configuration programming, cleared the fault codes, and the airbag light turned off.
Original source ↗
BYD DTC AI Analysis

BYD Qin Plus EV: Loose connector after accident repair

✓
2021 model year Qin Plus EV: minor front-end collision. After workshop repairs, the airbag warning light stayed on at startup, occasionally accompanied by intermittent horn failure. Retrieved DTCs B171811 and B171911. Inspected the driver-side airbag connector; found it not fully seated from the previous repair, with a broken locking tab. Checked continuity with a multimeter and found poor contact in the harness between the clock spring and airbag — resistance fluctuated between 5–15 Ω. Replaced the airbag harness connector with a repair kit, re-crimped the terminals, ensured the connector was fully seated, and fitted an anti-disengagement clip. Ran the 'SRS Configuration' function using special equipment. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM clock spring intermittent poor contact

✓
The airbag warning light on a 2019 Tang DM illuminated intermittently, especially when turning the steering wheel to full left lock or driving over bumpy roads. The light sometimes returned to normal when stationary. Using an X431 scan tool, the technician retrieved historical DTC B171811. He then measured the airbag circuit resistance while turning the steering wheel and found the resistance momentarily went to infinity at specific angles. He diagnosed poor contact in the clock spring’s internal slip ring due to long-term wear, causing intermittent open/short circuit faults. The technician replaced the multi-function switch/clock spring assembly (with angle sensor). During installation, he centred the new clock spring by turning it 2.5 turns to the left and then 2.5 turns to the right to return to centre. The fault did not recur after fitting.
Original source ↗
BYD DTC AI Analysis

BYD Yuan EV360 clock spring shorted after wading.

✓
After wading, the 2018 Yuan EV360's airbag warning light stayed on, and the driver's window and horn both stopped working. Fault codes showed B171811 (driver airbag circuit) and B171511 (horn circuit). Removed the lower steering column trim and found obvious water traces inside the clock spring connector, with the pins oxidised green. Measured the airbag circuit resistance at 12 ohms, well above spec. Water had entered through the steering column seal during wading, causing an internal short and then an open circuit in the clock spring. Replaced the clock spring assembly, cleaned the body harness connectors, and applied adhesive sealant to waterproof the steering column seal. After clearing the codes, the airbag, horn and window functions all returned to normal.
Original source ↗
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.