AU
B168400

DTC B168400 indicates the Occupant Classification System (OCS) detects an internal fault or abnormal signal — Seal U

Safety System

DTC B168400 indicates the Occupant Classification System (OCS) detects an internal fault or abnormal signal.

Installed inside the front passenger seat, this key subsystem of the SRS airbag system uses a pressure sensor matrix to monitor seat occupancy status in real time (empty seat/child/adult).

The OCS controls the front passenger airbag deployment strategy (suppression, low-power deployment, or full-power deployment) based on the occupant weight class (usually Class 0, Class 1, Class 2, or Class 3).

When this DTC triggers, the SRS ECU cannot accurately determine the front passenger occupant status.

The system enters fail-safe mode, disables the front passenger airbag by default (to prevent injury to children), and illuminates the airbag fault warning lamp.

This fault may prevent correct front passenger airbag deployment during a collision or cause unintended deployment when the seat is empty, severely compromising passive safety performance.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Internal fault in the front passenger seat OCS pressure sensor assembly (damaged sensor diaphragm, open strain gauge circuit, or damaged signal processing chip) typically resulting from long-term usage fatigue or excessive seat load.
  • 2Poor contact between the OCS control module and wiring harness connector, pin oxidation, or water ingress corrosion, common after driving through water, seat cleaning fluid leakage, or parking in a damp basement.
  • 3Short to ground, short to power, or open circuit in the OCS system power supply or CAN communication circuits, involving wiring harness wear at the dedicated yellow SRS connector under the seat.
  • 4Failure to perform the OCS calibration procedure after replacing the seat assembly, seat cushion foam, or seat frame causes the zero-point calibration data stored in the control module to mismatch the actual physical condition.
  • 5OCS control module software version defects or hardware aging cause abnormal pressure signal sampling. Some 2019-2020 E2/E3 production batches contain this software bug.
What To Do
  • 1
    Use the BYD VDS or DTS diagnostic tool to read the complete fault code stream, check for accompanying B168401-B1684FF sub-codes, and verify if the Occupancy Status and Weight Class values in the OCS live data stream display as 'Invalid' or 'Error'.
  • 2
    Turn off the ignition. Disconnect the negative battery terminal and wait 3 minutes for the SRS system capacitor to discharge. Visually inspect the yellow OCS connector under the front passenger seat for looseness, water ingress, or green corrosion on the pins. If necessary, clean the connector with electrical contact cleaner and apply conductive grease.
  • 3
    Remove the front passenger seat (4 retaining bolts, typical torque 40-45Nm). Inspect the OCS control module under the seat frame for physical damage, water stains, or burn marks. Measure the module power supply voltage (should be 12V±0.5V) and the CAN-H/CAN-L voltage (standard approx. 2.5V, differential signal 0-2V).
  • 4
    Measure the harness resistance (< 1 Ω) and insulation resistance (> 1 MΩ) between the OCS sensor and the SRS ECU using a multimeter. Inspect the harness sleeve at the moving parts of the seat slide rail for wear causing an intermittent short circuit.
  • 5
    If the wiring is normal, perform the OCS system calibration procedure: Remove all items from the seat. Use the diagnostic tool to enter 'SRS System Special Functions' and select 'OCS Calibration'. Follow the prompts to sequentially place 36kg (simulating a child) and 75kg (simulating an adult) standard weights. Wait for the system to learn and store the pressure curve.
  • 6
    If calibration fails or B168400 persists, replace the OCS sensor assembly (part number usually starts with 5A-; order with the seat frame or replace the sensor mat separately). After replacement, repeat the calibration procedure.
  • 7
    Restore all connections and reconnect the battery. Turn the ignition switch to ON and confirm the airbag warning lamp turns off after the self-check. Use the diagnostic tool to confirm the B168400 status changes to 'history fault' and clear the DTC. Perform a front passenger airbag simulated deployment test (use the diagnostic tool to execute the 'Component Test' function and confirm the system correctly identifies the occupant status).
Real-World Cases
BYD DTC AI Analysis

Water ingress into the seat caused an intermittent OCS module fault.

✓
A 2019 BYD E2 came in with the airbag warning light staying on after driving in the rain. The scan tool retrieved current fault code B168400. We found the passenger foot mat damp. Tracing the leak, we identified deterioration of the front windscreen's lower-right corner seal, which allowed water to run down the A-pillar and pool beneath the passenger seat. We disassembled and found obvious water staining inside the OCS module connector, with the pins oxidized green. We replaced the OCS control module (water had seeped internally, causing circuit board corrosion), repaired the windscreen seal, dried the seat frame, and recalibrated the OCS system to clear the fault. We advised the owner to regularly inspect the windscreen seal.
BYD DTC AI Analysis

Airbag warning light illuminated after seat replacement without calibration

✓
A 2020 BYD E3 displayed an airbag warning light after a third-party shop replaced the front passenger seat foam. I pulled fault codes B168400 and B168401 (OCS not calibrated). The original shop hadn't realised the OCS system needed recalibration. To fix it, I used VDS to access the SRS system and selected 'Special Functions' — 'OCS Zero Calibration'. I cleared all items from the seat to run zero-point learning, then placed 36 kg and 75 kg weights in sequence to complete full-range calibration. During calibration, the system flagged 'pressure sensor signal abnormal'. I found the replacement foam was too thick, causing the sensor's initial pressure reading to exceed the normal range. After adjusting the foam thickness, I ran the calibration again and it succeeded. The fault codes cleared and the system returned to normal.
BYD DTC AI Analysis

Wiring harness wear caused OCS communication interruption

✓
A 2019 Qin EV exhibited occasional flickering of the airbag warning light when driving on rough surfaces, while remaining normal when stationary. Diagnosis revealed intermittent DTC B168400, with live data showing the OCS status toggling between 'Occupied' and 'Error'. Inspection focused on the wiring beneath the seat, revealing wear marks on the transition harness between the OCS connector and body loom at the foremost seat rail position. Damaged insulation caused intermittent short-to-ground on the CAN-L line. Repaired the damaged harness by cutting out the affected section, soldering the wires, and insulating with heat-shrink tubing. Rerouted the loom to clear the seat rail travel path and secured it with cable ties to the wiring bracket, to prevent further wear during seat adjustment. Fault resolved.
BYD DTC AI Analysis

OCS sensor aging drift triggered the fault.

✓
A BYD E2 ride-share vehicle with 120,000 km came in with DTC B168400 and a constantly lit passenger seatbelt warning light, even when the seat was occupied. Live data showed the OCS weight reading was about 15 kg below actual weight, so the system interpreted the seat as 'empty'. We traced the fault to pressure sensor strain gauge fatigue from prolonged high-frequency use, causing output signal drift. We attempted the OCS calibration procedure, but the system returned 'Calibration Failed - Signal Out of Tolerance'. We replaced the OCS sensor assembly (including the pressure pad and control module), ran the standard calibration, and verified accurate weight detection at all levels using standard weights. This completely cleared the fault. We recommend high-mileage commercial vehicles have their OCS accuracy checked every 100,000 km.
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.