AU
B166800

DTC B166800 indicates a functional fault in the Left Side Impact Sensor within the Supplemental Restraint System (SRS) — Seal U

Safety System

DTC B166800 indicates a functional fault in the Left Side Impact Sensor within the Supplemental Restraint System (SRS).

This sensor typically mounts inside the left B-pillar trim panel or near the left side sill beam.

It contains a built-in accelerometer to detect side-impact acceleration on the left side of the vehicle.

When the sensor detects acceleration exceeding a specific threshold, it sends a crash signal to the SRS ECU, which then determines whether to deploy the left side airbag and side curtain airbag.

This DTC triggers when the SRS ECU continuously fails to receive a valid signal from the sensor (due to communication loss, abnormal signal voltage, or sensor unresponsiveness), disabling the left-side impact protection function.

In this fault state, the side airbag and side curtain airbag may fail to deploy during a side collision, severely endangering occupant safety.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose connection, water ingress, or terminal corrosion at the left side impact sensor wiring harness connector (common at the B-pillar, where frequent door opening and closing causes long-term harness bending).
  • 2Damaged accelerometer element inside the sensor body or signal processing circuit fault (due to aging, vibration, or electromagnetic interference)
  • 3Sensor power supply circuit (usually 5V reference voltage) short to ground, short to power, or open circuit; or signal circuit (LIN or CAN communication line) short or open circuit.
  • 4Loose, deformed, or detached sensor mounting bracket causing sensor misalignment or poor contact of internal components.
  • 5SRS ECU internal interface circuit fault; fails to correctly interpret the left-side crash sensor signal.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to read all fault codes and freeze frame data. Confirm vehicle speed, temperature, and other environmental parameters at the time of the fault. Check for related historical fault codes (such as B166711 short to ground or B166900 configuration error).
  • 2
    Switch off the ignition, disconnect the battery negative terminal, and wait at least 90 seconds (to allow the SRS capacitor to discharge). Remove the left B-pillar lower trim panel and visually inspect the left side impact sensor for physical damage, cracks, or loose mounting.
  • 3
    Disconnect the sensor connector. Check for backed-out terminals, corrosion, or water ingress, and measure the terminal contact pressure. Check the wiring harness at the B-pillar bend for signs of breakage or wear.
  • 4
    Reconnect the battery and turn on the ignition. Measure the voltage at the sensor connector power supply terminal (normal value: 5V±0.25V reference voltage or 12V power supply, depending on the specific model). Measure the ground wire resistance (should be less than 1Ω).
  • 5
    Measure the signal line voltage (LIN line is typically near 12V; CAN line should be approximately 2.5V reference). If the voltage is abnormal, check the wiring harness to the SRS ECU in sections for continuity and insulation.
  • 6
    If circuit measurements are normal, replace the left side impact sensor (Note: some models require sensor calibration or configuration coding). Tighten the sensor to the standard installation torque (usually 8-10 N·m) and secure it firmly.
  • 7
    Reconnect all connectors, clear the fault code, and use VDS to perform an SRS system self-check. Verify the left crash sensor status in the data stream displays 'Normal'. If necessary, perform a light tap test to verify sensor response.
  • 8
    Perform a road test simulating bumpy road conditions to confirm the fault indicator does not illuminate again and the system has no intermittent faults.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Poor contact in the left B-pillar wiring harness caused an intermittent fault on a Qin PRO DM.

✓
A 2019 BYD Qin Pro DM. The owner reported the airbag warning light coming on intermittently, particularly after opening or closing the left front door. Used VDS to retrieve DTC B166800. Freeze frame data showed the vehicle was stationary when the fault occurred. Diagnosis found the impact sensor connector below the left B-pillar was loose, with slight oxidation on the terminals. The wiring harness retaining clip had detached, causing the harness to flex and pull repeatedly during door operation. Repair: re-secured the harness routing, cleaned the connector with dedicated terminal cleaner, applied conductive grease to prevent oxidation, and fitted a new retaining clip. Post-repair follow-up over one month showed no recurrence.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro petrol: Sensor connector pin backed out after accident repair

✓
A 2018 Qin PRO petrol variant sustained left-side damage in a collision and was repaired at a workshop. After the repair, the airbag warning light stayed on. A VDS scan showed DTC B166800. An inspection of the left side impact sensor found the power terminal (pin 1) inside the wiring harness connector had backed out, cutting off the sensor power supply. The root cause: during the collision repair, the harness plug was forced in without proper alignment, damaging the terminal retaining spring and causing the terminal to back out. Diagnostic process: measuring the sensor connector terminals found no 5V supply voltage. Tracing the wiring harness revealed the terminal had backed out at the GJK14 junction. Solution: repaired the backed-out terminal, re-crimped and reinforced the retaining spring, and seated the connector fully until it clicked. After clearing the DTC, the system returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM: Sensor internal short circuit after wading

✓
A 2018 Qin Pro DM. After driving through water, the airbag warning light on the dashboard stayed on. VDS showed DTC B166800, which would not clear. The left carpet was wet. Tracing the leak found the left front sill seal had aged, allowing rainwater to get into the left side impact sensor. The sensor resistance measured 0.5Ω (normal is several thousand ohms), indicating an internal short. The wiring harness connector had water stains and verdigris. Replaced the left side impact sensor, cleaned the connector with special cleaner and blew it dry, replaced the sill seal, and added a waterproof silicone pad at the sensor mounting location. After repair, insulation resistance exceeded 10MΩ; fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro: Loose B-pillar sensor mounting bracket causing abnormal noise and a warning light

✓
A 2019 Qin Pro petrol variant. The owner reported a clicking noise from the left front B-pillar area on rough roads, along with an intermittent airbag warning light. We initially suspected a loose B-pillar trim panel. After removing it, we traced the noise to the side impact sensor itself. Inspection found the sensor mounting bracket bolts had loosened (torque: 2 N·m vs. 10 N·m spec), letting the sensor strike the body metal on bumps. Loose solder joints on the internal accelerometer were generating the fault signal. We re-tightened the bracket bolts to the standard torque and added a rubber damping pad between the sensor and bracket to eliminate vibration transmission. After clearing the fault codes, the noise and warning light disappeared.
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]