AU
B166600

DTC B166600 indicates a communication interruption between the SRS (Supplemental Restraint System) ECU and the left side impact sensor, or circuit resistance exceeding the calibrated range (typically >10kΩ or open circuit) — Seal U

Safety System

DTC B166600 indicates a communication interruption between the SRS (Supplemental Restraint System) ECU and the left side impact sensor, or circuit resistance exceeding the calibrated range (typically >10kΩ or open circuit).

On BYD E2/E3/Qin series models, the left side impact sensor mounts below the left B-pillar or on the left front door inner sheet metal to monitor left-side impact acceleration.

The ECU triggers this DTC and illuminates the airbag warning lamp upon detecting a continuous open sensor signal circuit, an open internal sensor circuit, or an abnormal sensor power/ground circuit.

Under this condition, side and curtain airbags may fail to deploy, but front airbag function usually remains unaffected.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Loose or oxidized left front door wiring harness connector: Frequent opening and closing of the door easily loosens the wiring harness connector installed near the door hinge, or water ingress after washing the car corrodes the terminals, resulting in poor contact or an open circuit in the signal line.
  • 2Side impact sensor internal open circuit: A damaged internal piezoelectric element or integrated circuit prevents the sensor from sending a crash signal to the SRS ECU. Typical causes include water ingress or sensor aging (vehicles over 5 years old).
  • 3Physical damage to the wiring harness: The seat track crushes, aftermarket audio wiring chafes, or incorrect routing during collision repairs damages the wiring harness inside the left front door sill trim or B-pillar, causing a signal wire short to ground or open circuit.
  • 4Poor contact at SRS ECU terminals: ECU connector not fully seated, terminal backed out, or terminal spread, preventing the ECU from recognizing the sensor resistance (normally approx. 2-3kΩ).
  • 5Sensor mounting bracket deformed: Incorrect sensor installation after accident repairs strains the connector and causes poor contact, or loose sensor retaining bolts cause poor grounding.
What To Do
  • 1
    Safety precautions: Disconnect the 12V battery negative terminal. Wait at least 90 seconds for the SRS capacitor to fully discharge to prevent accidental airbag deployment. Wear an anti-static wrist strap.
  • 2
    Fault Confirmation: Connect the VDS or Launch diagnostic tool. Read all DTCs and record the freeze frame data. Confirm the fault is current (Active) rather than historical. Check for accompanying communication fault codes (such as the B165X series).
  • 3
    Locate the sensor: Refer to the vehicle repair manual (E2/E3: inside the left lower B-pillar trim panel; Qin Pro: near the side impact beam inside the left front door). Remove the corresponding trim panel and locate the black or yellow side impact sensor connector.
  • 4
    Visual inspection: Verify the connector is fully seated. Inspect terminals for green corrosion or burn marks. Inspect the wiring harness at the door hinge for damaged insulation or broken wires. Measure wiring harness continuity between the connector and the ECU (should be <1Ω).
  • 5
    Electrical measurement: Disconnect the sensor connector. Measure the voltage between the signal wire and ground (normal: 5V reference voltage). Measure the sensor resistance (normal: 2-3kΩ; infinite resistance indicates a faulty sensor). If the wiring is normal but the sensor resistance is abnormal, replace the sensor.
  • 6
    Wiring repair: If the wiring harness has an open circuit, repair using waterproof connectors. Avoid direct twisted splices. If the connector has corrosion, clean with electrical contact cleaner and apply conductive grease. After repair, verify the wiring harness is the correct length to prevent the door from pinching it.
  • 7
    Verification test: Reconnect all connectors, reconnect the battery, and clear the fault code. Perform a static test (ignition switch ON; the airbag warning lamp must turn off during the instrument cluster self-check). Perform a dynamic test (lightly tap the vehicle body near the sensor and monitor the data stream for acceleration signal changes).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro DM airbag warning light stays on after car wash

✓
A 2019 Qin Pro DM. The owner reported the airbag warning light staying on after washing the vehicle. Used VDS to retrieve DTC B166600 (current fault). Removed the left B-pillar lower trim and found water traces inside the side impact sensor connector; terminals oxidised and blackened. Cause: aged window weatherstrip allowed high-pressure water to enter the B-pillar during washing, corroding the sensor connector and causing poor contact. Cleaned the terminals with electronic cleaner, applied waterproof silicone grease, and replaced the weatherstrip. Fault resolved. Advised the owner to avoid using a high-pressure washer directly on the B-pillar area.
BYD DTC AI AnalysisFrom Chinese market (translated)

Side airbag warning light comes on after E2 accident repairs.

✓
After a left-side collision, a 2019 BYD e2 was repaired at a workshop. On delivery, the instrument cluster showed an airbag warning. Diagnosis found DTC B166600. Inspection revealed that when replacing the left front door, the technician had not routed the side impact sensor harness correctly through the waterproof grommet, causing it to be pinched in the door hinge. The internal copper conductors were fractured, but the outer insulation was intact. The wiring was re-routed using a dedicated harness protection sleeve, the open circuit repaired, and the fault resolved. Reminder: during collision repairs, pay close attention to harness routing to avoid mechanical pinching.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV side impact sensor internal open circuit

✓
A 2019 BYD Qin EV with 80,000 km had an intermittent airbag warning light. The fault returned a few days after clearing the DTCs. Live data showed the left side impact sensor signal occasionally reading 255 (out of range). Static resistance measurement of the sensor sometimes showed infinite resistance. Replacing the left side impact sensor (part number: E2A-5820400) permanently fixed the fault. Disassembling the old sensor revealed a dry solder joint on the internal PCB—a manufacturing defect. Use genuine parts when replacing this sensor. Aftermarket sensors may have resistance values outside the calibration range.
BYD DTC AI AnalysisFrom Chinese market (translated)

E3 aftermarket stereo installation triggered SRS fault

✓
A 2020 E3 had the SRS warning light illuminate after the owner modified the four-door audio system. DTC B166600 was stored. Inspection revealed that when routing the left front door speaker wiring harness, the installer used self-tapping screws to secure it. One screw penetrated the factory wiring conduit, piercing the side impact sensor signal and earth wires, causing an intermittent short. Repaired the damaged harness, re-routed it properly, and secured it with cable ties (avoiding metal screws). Fault resolved. Reminder: protect factory wiring harnesses during audio modifications. SRS wiring is typically enclosed in yellow conduit and must not be altered.
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]