AU
B1666-00

DTC B1666-00 indicates the Airbag Control Unit (ACU) detected a communication loss or abnormal signal from the Left Side Impact Sensor (LSIS) — Qin Plus

Safety System

DTC B1666-00 indicates the Airbag Control Unit (ACU) detected a communication loss or abnormal signal from the Left Side Impact Sensor (LSIS).

This sensor, typically located in the left B-pillar or left front door area, monitors vehicle side-impact acceleration.

Root causes include: 1) Open circuit in the sensor power or ground lines; 2) Open circuit, short circuit, or short to ground in the communication harness (LIN bus or hardwire); 3) Failure of the sensor internal piezoelectric accelerometer or signal processing circuit; 4) Incorrect sensor installation causing the signal to fall outside the calibrated range.

This safety-critical fault may prevent the left side airbag and curtain airbag from deploying during a side impact, or cause unintended deployment during normal driving.

4
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Loose connectors, backed-out pins, or oxidation/corrosion in the left front door wiring harness or B-pillar wiring harness, especially after frequent door operation or wading.
  • 2Internal fault in the left side impact sensor, such as a damaged accelerometer element or open internal circuit.
  • 3Physical damage to the wiring harness, including pinched or broken wires after accident repairs, or broken copper strands caused by prolonged bending.
  • 4Deformed sensor mounting bracket or loose retaining bolts causing poor grounding between the sensor and vehicle body, or mounting angle deviation.
  • 5Internal interface circuit fault in the Airbag Control Unit (ACU) preventing sensor signal recognition (less common, but requires inspection)
What To Do
  • 1
    Use the dedicated diagnostic tool to read all fault codes and freeze frame data. Check for accompanying B1667 (right side impact sensor) or other SRS fault codes, and verify the vehicle status when the fault occurred.
  • 2
    Disconnect the battery negative terminal and wait 3 minutes. Visually inspect the left side impact sensor (located inside the left B-pillar trim panel or behind the left front door trim panel) and its wiring harness connector. Check for water stains, corrosion, or physical damage.
  • 3
    Measure the sensor connector terminal voltage: check the power supply terminal (usually 12V or 5V, depending on vehicle model), ground terminal (<0.5Ω to body ground), and signal wire voltage (LIN line fluctuates between 7-11V; hard-wired type uses a 2.5V reference).
  • 4
    Measure sensor resistance: Disconnect the connector and measure the sensor resistance. The standard value is typically 2-3 kΩ (refer to the workshop manual). If open or shorted, replace the sensor.
  • 5
    Perform a wiring harness continuity test: Test the continuity of the wiring harness between the sensor connector and the ACU connector. Inspect the harness at the door hinge for breaks. Required resistance: <1Ω.
  • 6
    Check sensor installation status: Confirm the sensor installation direction arrow points to the front of the vehicle, the fastening torque meets the standard (usually 8-10 N·m), and the mounting surface is free of foreign matter.
  • 7
    Repair or replace: Based on inspection results, repair the wiring harness, clean the connectors, or replace the sensor. Reconnect all components and restore battery power.
  • 8
    System reset and verification: Clear the fault code. Turn the ignition switch to the ON position and verify the SRS indicator turns off after the self-check. Use a diagnostic tool to read the data stream and confirm the left crash sensor real-time acceleration value displays normally (close to 0G when stationary).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Broken left front door wiring harness on Qin EV300 causing B1666-00

✓
Vehicle: 2017 Qin EV300, 80,000 km. Fault symptom: Airbag warning light on instrument panel stays on. Retrieved fault code B1666-00. Inspection found the wiring harness at the left front door hinge had internal copper wires broken due to long-term door opening and closing, with no visible external damage. Stripping back the insulation revealed three wires completely severed. Repair: Soldered the broken wires and sealed with heat-shrink tubing, then rerouted the harness to avoid excessive bending. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Water ingress caused corrosion in the BYD E1 side impact sensor connector

✓
Vehicle: 2019 E1. Fault appeared after heavy rain. Symptom: Airbag warning light came on after driving through water. Diagnostic scan showed DTC B1666-00. Inspection found the side impact sensor connector beneath the left B-pillar had visible water stains and green copper corrosion inside, causing the signal wire to short to ground. Repair: Thoroughly cleaned the connector contacts with electronic cleaner, applied conductive grease, replaced the waterproof seal, and waterproof-wrapped the wiring harness. The fault has not recurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin 100 sensor incorrectly fitted following accident repair

✓
Vehicle: 2017 Qin 100, after left-side collision repair. Symptom: Airbag warning light stayed on after repairs, DTC B1666-00. Cause: When replacing the left front door, the technician did not install the side impact sensor correctly. The sensor mounting bracket was deformed, and the sensor was installed backward (arrow pointing rearward). Fix: Replaced the sensor bracket, reinstalled the sensor in the correct orientation with specified torque, and ran the "collision sensor calibration" procedure using a diagnostic tool. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Intermittent open circuit inside the Qin EV450 sensor

✓
Vehicle: 2018 BYD Qin EV450, intermittent fault Symptom: Airbag warning light illuminates intermittently, usually on rough roads, and turns off when stationary Diagnosis: Used a diagnostic scan tool to monitor live data; left side impact sensor signal occasionally became ‘invalid’. Checked the wiring harness — normal. Measured sensor resistance — normal when cold, but an open circuit occurred when lightly tapping the sensor body. Repair: Replaced the left side impact sensor (part number: ...). Fault completely resolved.
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. Sources: [1]