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B166A00

DTC B166A00 indicates interrupted communication or abnormal data between the airbag control unit (SRS ECU) and the Left Side Impact Sensor — Seal U

Safety System

DTC B166A00 indicates interrupted communication or abnormal data between the airbag control unit (SRS ECU) and the Left Side Impact Sensor.

This sensor typically mounts at the lower B-pillar or under the left front seat and monitors left-side impact acceleration.

A communication error prevents the ECU from receiving real-time impact data from the sensor, causing the side and curtain airbags to fail to deploy accurately during a collision, or risking unintended deployment.

This fault constitutes an active safety system failure.

The vehicle remains drivable, but side-impact protection functions degrade.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Left side impact sensor wiring harness connector loose, pins backed out, or oxidized/corroded; commonly occurs after wading, car washes, or failing to fully seat the connector after collision bodywork repairs.
  • 2Internal fault in the sensor body, including MEMS accelerometer chip damage, internal water ingress, or dry solder joints from prolonged vibration.
  • 3Wiring harness short or open circuit, especially sill wiring harness damage from seat rail compression, rodent damage, or aftermarket modifications (such as installing illuminated scuff plates or ambient lighting).
  • 4Internal LIN/CAN transceiver fault in the airbag ECU preventing parsing of the left-side sensor data frame (less common, usually occurs with other communication fault codes)
  • 5Sensor mounting position misaligned or mounting bracket deformed, causing poor connector contact (typically after chassis bottoming out or side collisions).
What To Do
  • 1
    Use the VDS or X431 diagnostic tool to read all SRS system fault codes. Confirm whether B166A00 is an active or history fault and record the freeze frame data.
  • 2
    Visually inspect the instrument panel airbag warning light status to confirm the lamp circuit operates normally during the system self-check (the light should turn off 6 seconds after turning the ignition switch ON; if it remains illuminated, the fault is present).
  • 3
    Remove the left lower B-pillar trim panel or left front door sill trim panel, locate the left side impact sensor (usually marked 'Left Side Impact' or 'SIS-L'), verify the connector is fully locked, and inspect the pins for green corrosion, backed-out pins, or signs of water ingress.
  • 4
    Disconnect the battery negative terminal and wait 90 seconds for the capacitor to discharge. Measure continuity at the sensor wiring harness terminals: check the power supply (B+), ground (GND), and LIN line (or CAN-H/CAN-L) for continuity. The LIN line voltage to ground should be approximately 11V (ignition ON).
  • 5
    Perform a swap test: Swap the left and right side impact sensors. Clear and recheck the fault codes. If the fault code changes to B167000 (right side impact sensor communication error), the sensor is faulty. If the fault code remains B166A00, inspect the wiring harness or ECU.
  • 6
    Check wiring harness continuity from the SRS ECU (usually under the centre armrest or inside the centre console) to the left sensor. Focus on the connector between the floor harness and the body harness (usually above the left footrest).
  • 7
    If the wiring harness is normal, replace the OEM left side impact sensor (part number usually starts with 5A-363). Install the sensor with the arrow pointing toward the front of the vehicle and tighten to 8-10 N·m.
  • 8
    Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code and execute the 'Crash Sensor Configuration' or 'Coding' procedure (some models require online matching). Finally, perform a system self-check to confirm the fault is resolved.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM lost communication with the left sensor after wading.

✓
A 2019 BYD Qin Pro DM. Owner reported the airbag warning light stayed on constantly after driving through floodwater during a heavy rain. Scan tool read active fault code B166A00. Removed the left B-pillar trim panel and found clear water stains and green corrosion inside the sensor connector. Cleaned the connector pins with WD-40 Precision Contact Cleaner, applied conductive grease, and reassembled. The fault temporarily cleared but returned after one week. Finally replaced the wiring harness connector and sensor to fix the issue. For vehicles that have waded through water, check the floor wiring harness sealing carefully.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro (petrol): Sensor not configured after accident repair

✓
2018 BYD Qin Pro petrol. The airbag warning light came on after left-side accident repairs. DTC B166A00 stored. Inspection found the left front door and B-pillar sheet metal were replaced during the accident repair. The technician reinstalled the side impact sensor but forgot to connect the connector. After connecting the connector, the DTC remained. Used the BYD diagnostic tool to run the 'SRS Configuration' function, performed address configuration on the left sensor (wrote the configuration code), and cleared the DTC. Online configuration is mandatory after sensor replacement.
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket ambient lighting caused LIN bus short circuit

✓
2020 BYD Qin Pro DM. The airbag warning light illuminated the day after the owner had interior ambient lighting fitted at the dealership. DTC B166A00. Inspection found that while routing wiring through the sill, the modification technician pierced the LIN communication line to the left impact sensor when securing a wiring clip with a screw, causing a short to earth. Repaired the damaged wiring and insulated it with heat-shrink tubing. Fault cleared. Recommendation: BYD SRS wiring is highly integrated with the body harness; avoid drilling holes or mounting foreign objects in the sill area during modifications.
BYD DTC AI AnalysisFrom Chinese market (translated)

Intermittent communication failure due to sensor aging

✓
A 2018 Qin Pro DM with 120,000 km on the clock had an intermittent airbag warning light. Fault code B166A00 was intermittent. The wiring harness and connectors looked normal, but resistance at the sensor sometimes read open circuit. Replaced the left side impact sensor, but the fault remained. Finally found the sensor mounting bracket was loose, causing poor connector contact. Re-secured the bracket and added anti-vibration washers to fix the issue. On older vehicles, check the sensor bracket for deformation from chassis vibration.
Other Seal U 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]