AU
B165400

DTC B165400 indicates the airbag control unit (SRS ECU) detects a communication interruption with the Left Front Impact Sensor during a self-test or drive cycle — Seal U

Safety System

DTC B165400 indicates the airbag control unit (SRS ECU) detects a communication interruption with the Left Front Impact Sensor during a self-test or drive cycle.

This sensor typically mounts on the left front longitudinal member (inside the fender or near the headlamp bracket) and detects front collision acceleration signals. "Not connected" indicates a disconnected physical plug, an open circuit in the sensor power/ground wiring, an open CAN/LIN communication line, an internal sensor open circuit, or a poor connection at the ECU connector.

An active fault forces the SRS system into a degraded mode.

This condition may prevent the left front airbag, driver airbag, or seatbelt pretensioner from deploying correctly during a collision, and illuminates the instrument cluster airbag warning light (flashing or solid).

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1After front-end accident repairs, the left front crash sensor connector is not fully seated, or a broken locking tab causes a loose connection. This commonly occurs after removing and installing the front bumper, headlight, or fender.
  • 2Sensor wiring harness wear and open circuit. This mostly occurs in the front longitudinal beam area. Bottoming out, wading, or prolonged vibration damages the harness insulation and breaks the internal copper wires, especially due to protective sleeve aging where the harness passes through the front longitudinal beam hole.
  • 3Internal sensor fault, such as a damaged piezoelectric element, internal open circuit, or ID chip failure, preventing effective communication with the SRS ECU.
  • 4Poor contact at the SRS control unit connector or internal ECU fault prevents the specific channel from detecting sensor resistance (normally approx. 2.0-3.0 kΩ).
  • 5Accidental cutting or damage to the collision sensor wiring harness during vehicle modification or installation of non-original equipment (such as front radars or spotlights), or a deformed sensor mounting bracket causing the connector to loosen under stress.
What To Do
  • 1
    Connect the VDS2000/Launch X431 diagnostic tool, access the SRS system, and read the complete fault codes. Confirm whether B165400 is a current fault (Active) or a history fault (History). Clear the fault code, perform a road test or simulated vibration test, and observe if the fault returns. Simultaneously record the Freeze Frame data to check the vehicle speed and ambient temperature when the fault occurred.
  • 2
    Disconnect the battery negative terminal (follow the high-voltage power-down procedure; for hybrid models, ensure the READY indicator is off) and wait 90 seconds for the capacitors to discharge. Remove the left front fender liner or front bumper. Visually inspect the 2-pin or 3-pin connector of the left front crash sensor (located at the front of the left front longitudinal beam or below the headlight) for a secure connection. Check the locking tab for breakage and inspect the terminals for backing out, corrosion, or white oxidation from water ingress.
  • 3
    Test the sensor wiring harness using a multimeter: disconnect the sensor connector and measure the voltage to ground on the harness side (expected value: 0V; abnormal voltage indicates an ECU or wiring short circuit). Measure the resistance between the sensor power wire and ground (must be less than 1Ω). Use the multimeter continuity setting to check harness continuity. Inspect specifically for hidden open circuits inside the harness protective sleeve at the front longitudinal beam pass-through. If equipped with dual sensors, swap the left and right sensors to determine if the fault moves with the sensor.
  • 4
    Measure sensor body resistance: Disconnect the sensor and measure the resistance between its terminals using a multimeter. Normal resistance is 2.0 kΩ ±10% (refer to the repair manual for details). Infinite or zero resistance indicates an internal open or short circuit; replace the sensor. Install a genuine replacement sensor and tighten to the specified torque (typically 8-10 N·m). Do not use aftermarket or salvaged parts, as these can cause an ID mismatch.
  • 5
    After completing the repair, reconnect the battery and turn the ignition switch to ON. Use the diagnostic tool to execute 'SRS System Configuration' or 'Sensor ID Registration' (required for some Qin Pro and E2/E3 models). Clear the fault codes and perform an 'SRS System Self-check'. Finally, perform a vehicle crash simulation test (using the dedicated test tool) or a road test to confirm the fault code does not return and the airbag warning lamp turns off normally (6 seconds after the self-check).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM airbag warning light stays on after accident repair

✓
A 2019 BYD Qin Pro DM had its front bumper and left headlight replaced at a repair shop after a minor front collision. Afterwards, the airbag warning light on the instrument panel stayed on constantly. The stored DTC was B165400 (current). Inspection found the collision sensor connector on the left front side member was not fully seated and its locking tab had broken. The repair shop technician, unfamiliar with the BYD SRS system, pulled on the wiring harness while fitting the bumper, causing the connector to come loose. Replaced the wiring harness connector (with harness assembly), reconnected the sensor, cleared the fault codes, and performed a VDS system self-test. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD E2 left front sensor communication fault after water ingress

✓
2020 E2 EV. After driving through floodwater during heavy rain, the airbag warning light illuminated. The scanner logged DTCs B165400 and B165500 (right front) simultaneously. Inspection found damage to the wiring harness protective sleeve beneath the left front chassis rail in the front compartment. Water ingress corroded the sensor connector; the oxidized pins produced green copper deposits. Cleaned the connector with WD-40 precision electrical contact cleaner. Wiring harness continuity tested normal, but the sensor internals had short-circuited due to moisture. Replaced the left front impact sensor and waterproofed the harness using heat shrink tubing and waterproof sealant. Fault rectified. Warning: EV wading depth must never exceed half the tyre height.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV sensor wiring harness underbody scrape wear causes intermittent fault.

✓
2019 BYD Qin EV. The owner reported the airbag warning light illuminated intermittently, especially on rough roads. DTC B165400 was an intermittent historical fault. We raised the vehicle and inspected the airbag crash sensor wiring harness beneath the left front longitudinal rail. Where the harness passes through the rail opening, an underbody impact had worn through the protective sleeve. Only 2–3 copper strands remained connecting the wires, causing intermittent electrical contact while driving. We cut out the damaged harness section, crimped new waterproof terminals, insulated with double-layer heat shrink tubing, and re-routed the harness to prevent chafing against the rail edge. The fault did not return.
BYD DTC AI AnalysisFrom Chinese market (translated)

E3 front radar installation incorrectly modified collision sensor wiring

✓
2020 E3. After the customer had front parking sensors installed at an external workshop, the airbag warning light stayed on continuously when starting the vehicle. DTC B165400. Inspection found the retrofit shop mistakenly tapped into the left front collision sensor wiring harness, thinking it was the radar power wire. This caused a short circuit in the sensor circuit, and the SRS ECU protectively disabled that channel. Repaired the wiring harness (removed the wire taps, soldered the wires, and sealed with heat-shrink tubing). Replaced the damaged collision sensor (internally shorted). Recoded the SRS ECU to unlock the channel (the ECU had locked it after detecting the short). Fault resolved. Advised the customer to have accessories fitted at an authorised dealer.
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]