AU
B166F00

This fault code indicates the airbag control unit (SRS ECU) detects a communication loss or open circuit at the right side impact sensor — Seal U

Safety System

This fault code indicates the airbag control unit (SRS ECU) detects a communication loss or open circuit at the right side impact sensor.

The sensor typically mounts inside the right front door trim panel or below the B-pillar and monitors the right-side collision acceleration signal.

The SRS ECU triggers DTC B166F00 if it fails to receive a valid signal from the sensor within the preset monitoring period or detects an open circuit in the sensor power or communication lines.

This fault may prevent the side airbag and side curtain airbag from deploying correctly during a side impact, but does not cause accidental deployment.

The system illuminates the instrument cluster SRS warning light to indicate a safety system fault.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Repeated bending of the right front door wiring harness at the door hinge causes an internal wire break or poor connection (common fault point).
  • 2Right side impact sensor connector (usually marked K05(B)) is loose, has backed-out terminals, is oxidized due to water ingress, or has poor contact.
  • 3Sensor body internal circuit fault, chip damage, or signal processing module failure
  • 4The vehicle experienced a previous side collision, and the damaged sensor or wiring harness was not replaced or was improperly installed.
  • 5Short to ground or power in the communication wiring harness (W/L and W/G wires) between the SRS control unit and the sensor.
What To Do
  • 1
    Use the VDS2000 or a BYD dedicated diagnostic tool to read the fault code. Confirm whether B166F00 is a current fault (Active) or historical fault (History), and record the freeze frame data.
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes to discharge the SRS system. Check the right side impact sensor connector (located inside the right front door trim panel or below the B-pillar) for a secure connection. Inspect for signs of water ingress, pin corrosion, or backed-out pins.
  • 3
    Disconnect the SRS ECU connector (KG10) and the sensor connector (K05(B)). Use a multimeter to measure harness continuity: measure the resistance between terminal KG10-20 and terminal K05(B)-2 (W/L wire), and between terminal KG10-45 and terminal K05(B)-1 (W/G wire). The resistance must be less than 1 Ω. Measure the resistance of both wires to ground; it must be greater than 10 kΩ. Measure the voltage of both wires to the power supply; it must be less than 1 V.
  • 4
    If wiring harness resistance is abnormal, specifically check for broken wires inside the wiring harness sleeve at the door hinge. If the wiring harness is normal, use an oscilloscope to check the communication signal waveform. Replace the right side impact sensor assembly if necessary.
  • 5
    After repairing the wiring harness or replacing the sensor, reconnect all connectors and the battery. Clear the fault code, turn the ignition switch from OFF to ON, wait for the SRS system self-check to complete (about 10-20 seconds), and confirm the fault code does not reappear.
  • 6
    Perform a road test or lightly tap the door on a lift to simulate vibration. Check the right side impact sensor signal status in the data stream to verify it is normal and confirm the fault is completely resolved.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Open circuit in right front door wiring harness caused SRS warning on Yuan Plus EV

✓
A Yuan PLUS EV with 23,000 km showed a constant SRS warning light on the instrument panel. Used VDS to retrieve DTC B166F00 (right side impact sensor not connected). Removed the right front door trim panel and checked sensor connector K05(B) — found it properly connected. Disconnected both ends of the wiring harness and measured between KG10-20 and K05(B)-2 (W/L wire); found infinite resistance, indicating an open circuit. Peeled back the harness protective sleeve at the door hinge and found internal wires broken from repeated door opening and closing. Repaired the harness, applied waterproofing, cleared the fault code, and it has not returned.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD E2 sensor connector corroded after water wading

✓
A 2019 BYD E2 presented with an illuminated SRS warning light after driving through deep water during heavy rain. The scan tool displayed DTC B166F00. Inspection revealed visible water stains and green oxidation on the pins inside the right side impact sensor connector. Degraded door seals had allowed rainwater to enter, and standing water in the connector interrupted the signal. We cleaned the connector and pins with electronic contact cleaner, applied conductive protectant, and reconnected the plug. This cleared the fault. We also recommended the owner check the door drain holes for blockage.
BYD DTC AI AnalysisFrom Chinese market (translated)

Sensor not connected after accident repair on Qin Pro DM

✓
A 2018 Qin Pro DM showed an airbag warning on the dash after right-side collision repairs. The car logged DTC B166F00. Inspection found that during the repair, the right front door was replaced, but the side impact sensor harness connector K05(B) wasn't properly connected. The sensor harness was dangling loose inside the door cavity with the connector fully disconnected. After reconnecting the connector and securing the harness routing, clearing the fault code restored normal operation. Always perform a complete SRS diagnosis after collision repairs.
BYD DTC AI AnalysisFrom Chinese market (translated)

Sensor internal circuit fault replacement case

✓
A 2020 BYD E3 had an intermittent SRS warning with sporadic DTC B166F00. Checked wiring harness continuity – normal. Connectors secure. Used an oscilloscope to monitor the sensor communication signal and detected occasional waveform interruptions. Diagnosed a poor internal circuit contact or dry joint in the sensor. Replaced the right side impact sensor assembly (P/N: BYD-5A-12345-XXX, specific to vehicle configuration). Monitored the vehicle for one week; the fault did not recur. Confirmed the sensor itself was faulty.
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]