AU
B166F

DTC B166F indicates a loss of communication between the SRS (Supplemental Restraint System) ECU and the Right Side Impact Sensor (RSIS) — Seal U

Safety System

DTC B166F indicates a loss of communication between the SRS (Supplemental Restraint System) ECU and the Right Side Impact Sensor (RSIS).

This sensor typically mounts below the B-pillar on the right front door frame or the lower section of the right centre pillar to detect side-impact acceleration on the right side of the vehicle.

The SRS ECU sets this fault code if it fails to receive a signal from the sensor within a predetermined time, or detects circuit resistance outside the normal range (open/short circuit).

This fault may prevent the right side airbag and right curtain airbag from deploying correctly during a side impact, severely compromising occupant side-impact protection.

It does not affect frontal impact protection.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Poor connector contact: Right collision sensor connector K05(B) and body wiring harness connector KG10 are not fully locked, retaining clip is damaged, or terminals are oxidized, interrupting signal transmission.
  • 2Wiring harness open circuit or wear: Long-term vibration at the wiring harness retaining clip inside the B-pillar trim panel damages the insulation and partially breaks the wire, or pinching the harness during repairs causes an intermittent open circuit.
  • 3Sensor body damaged: Vehicle collision impact, water ingress, or electrical overload damaged the sensor's internal piezoelectric accelerometer element or signal processing circuit, resulting in an open circuit.
  • 4Incorrect installation: Installing the sensor in the wrong direction after removal (the arrow must point toward the front of the vehicle), insufficient fixing bolt torque (standard: 8-10 N·m), or mounting surface deformation causing poor contact between the sensor and the vehicle body.
  • 5SRS ECU terminal fault: Terminal 20 (W/L wire) or 45 (W/G wire) of ECU-side connector KG10 backed out or corroded, causing poor contact with the wiring harness.
What To Do
  • 1
    Safety preparation: Set the power mode to OFF, disconnect the negative battery cable, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Locate the component: Remove the right B-pillar lower trim panel, locate the right side impact sensor (usually marked RSIS or SIS-R), and verify the connection status of connector K05(B).
  • 3
    Visual inspection: Verify the connector is fully seated and the locking tab is intact. Inspect the wiring harness at the B-pillar trim clip for wear, cuts, or crush marks.
  • 4
    Circuit continuity test: Use a multimeter to measure the resistance from body harness side KG10-20 (white/blue wire) to sensor side K05(B)-2, and from KG10-45 (white/green wire) to K05(B)-1. Resistance should be <1Ω. Measure the resistance to ground; it should be >10MΩ (to rule out a short circuit).
  • 5
    Sensor body inspection: Disconnect the connector and measure the resistance between the two sensor terminals. Normal value is 2-3kΩ (at 24±5°C). If resistance is infinite or zero, replace the sensor.
  • 6
    Cross-check: Swap the left and right side impact sensors. If the fault code transfers to the left side (B166A), the sensor is faulty. If the fault remains, inspect the wiring.
  • 7
    Repair procedure: When repairing the wiring harness, solder using 0.5mm² twisted shielded wire and insulate with heat shrink tubing; if the connector is loose, repair the retaining clip or apply conductive grease; if the sensor is damaged, replace with a new part, point the arrow forward, and tighten to the specified torque of 8-10N·m.
  • 8
    Verification test: Connect the battery. Set the power to ON. Use the diagnostic tool to clear the fault code. Wait 20 seconds for the system self-check. Confirm the SRS warning light turns off. Read the data stream to confirm the right-side sensor acceleration signal is normal.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Loose connector on BYD Yuan Plus EV caused B166F fault

✓
Symptoms: Airbag warning light stays on after vehicle start. Retrieved DTC B166F (right side impact sensor disconnected). No collision history, but right B-pillar trim was recently removed for modification. Diagnosis: Found connectors K05(B) and KG10 not fully latched. Harness continuity measured between KG10-20 and K05(B)-2, and KG10-45 and K05(B)-1: normal (<1Ω). After reconnection the fault disappeared temporarily but returned, indicating poor contact. Resolution: Used a special tool to repair the connector locking tab, ensuring a positive lock. Applied conductive paste to enhance contact. Fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn wiring harness open circuit causing intermittent B166F-00 fault on BYD Qin

✓
Symptom: Airbag warning light illuminates intermittently when driving on rough roads. DTC B166F-00 stored as intermittent. Diagnosis: Visual inspection showed no external damage to the sensor. Sensor resistance measured 2-3kΩ, within specification. Found wiring harness insulation chafed through at the B-pillar trim clip, with internal conductors partially fractured. Flexing the harness caused resistance to fluctuate from <1Ω to open circuit, confirming an intermittent open circuit. Repair: Cut out the damaged section. Spliced in matching 0.5mm² twisted pair shielded cable, soldered the connections, and sealed with heat-shrink tubing. Secured the harness and added corrugated tubing for protection. Fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Sensor body damage caused B166F fault in BYD Yuan Plus EV.

✓
Symptoms: After a minor scrape on the right side (airbags did not deploy), the airbag warning light stays on constantly with DTC B166F. Diagnosis: The connector is secure and the 5V supply voltage is normal. The sensor's internal resistance measures infinite (specification: 2–3 kΩ), indicating the impact damaged the internal piezoelectric element or circuit board. Swapping the left sensor to the right side shifted the fault code to the left, confirming the sensor itself failed. Resolution: Replaced the right side impact sensor assembly, tightening the mounting bolt to 8–10 N·m with the arrow pointing forward. Cleared the DTC; system self-test returned normal.
Original source ↗
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]