AU
B1684-00

DTC B1684-00 indicates a functional fault in the Occupant Classification System (OCS) — Atto 3

Safety System

DTC B1684-00 indicates a functional fault in the Occupant Classification System (OCS).

The OCS is a key subsystem of the Supplemental Restraint System (SRS).

Located inside the front passenger seat, the system uses a pressure sensing mat and an electronic control module to monitor the seat load status in real time (empty seat/child/adult).

It sends classification signals to the SRS ECU to determine the front passenger airbag deployment strategy during a collision (deployment decision, force, and timing).

This DTC triggers when the OCS and SRS ECU lose communication, a sensor signal exceeds its threshold, the system loses calibration data, or a hardware failure occurs.

This fault forces the airbag system into fail-safe mode.

During a collision, this condition may cause unintended airbag deployment (injuring a child) or failure to deploy (loss of protection).

This poses a serious safety risk and requires immediate repair.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Physical damage or aging of the front passenger seat OCS pressure sensing mat (commonly resulting from water or liquid ingress into the seat, prolonged pressure from heavy objects, or pet scratching causing the sensing mat to break)
  • 2Loose, oxidized, backed-out pins, or poor contact at the OCS control module wiring harness connector (plug wear caused by frequent seat fore/aft movement or height adjustment)
  • 3Failure to perform OCS system zero-point and weight calibrations after accident repairs, seat removal/installation, or replacement; or an interrupted calibration process resulting in abnormal data.
  • 4OCS control module internal circuit board fault (ECU hardware damage, capacitor leakage, or chip software error)
  • 5Open circuit, short circuit, or signal interference in the CAN communication line between the SRS airbag control unit and the OCS module
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000/3000) to read SRS system fault codes, confirm B1684-00 is a current active fault, record freeze frame data, and check for related fault codes such as B1685 (OCS sensor fault).
  • 2
    Inspect the OCS module connector under the front passenger seat (usually a yellow waterproof plug). Verify the locking tab engages fully. Inspect the pins for oxidation, push-out, or water ingress. Measure the supply voltage (12V ± 0.5V), ground resistance (<1Ω), and CAN-H/CAN-L line voltages (approximately 2.5V).
  • 3
    Remove the front passenger seat and visually inspect the OCS pressure sensing mat beneath the seat foam cushion for damage, creases, foreign object punctures, or signs of liquid corrosion. Check the wiring harness between the sensing mat and the OCS ECU for breaks.
  • 4
    Perform OCS system calibration: Access the SRS system special functions on the diagnostic tool, select "OCS Calibration", and follow the prompts to sequentially complete the empty seat calibration, calibration by placing a 36kg standard weight (simulating a child), and calibration by placing a 75kg standard weight (simulating an adult). Verify each step displays "Calibration successful".
  • 5
    If calibration fails or the fault code will not clear, replace the OCS sensor assembly (for Qin series models, this usually requires replacing the entire front passenger seat cushion assembly or the OCS sensing mat module). After replacement, perform the complete calibration procedure and write the VIN.
  • 6
    Reinstall all components, clear the fault code, and perform an SRS system self-check cycle (cycle the ignition switch ON-OFF three times). Confirm B1684-00 does not reappear and the instrument cluster airbag warning light turns off. Use the diagnostic tool to read the OCS live data stream to verify accurate empty seat/occupied status recognition.
Real-World Cases
BYD DTC AI Analysis

Qin EV300 OCS sensor pad short circuit fault after water wading

✓
Symptoms: After heavy rain, the airbag warning light on the dashboard stayed on. Scan tool read active DTC B1684-00. Diagnosis: Inspection found water had entered above the door sills, wetting the passenger-side carpet. Removing the seat revealed water in the OCS pressure sensor mat connector. The sensor mat showed signs of electrolytic corrosion, and reduced insulation resistance caused signal errors. Repair: Dried the seat frame thoroughly and replaced the OCS pressure sensor mat assembly (part number: SA-5820130). Performed three-stage calibration (empty seat / 36 kg / 75 kg). Fault cleared.
BYD DTC AI Analysis

Fault due to not calibrating after accident repair on Qin 100.

✓
Symptoms: After front collision repairs, the airbag warning light stays on and DTC B1684-00 cannot be cleared. Diagnosis: The technician replaced the front passenger seat assembly but did not perform the OCS system calibration. The SRS ECU detected an OCS data format mismatch. Repair: Entered the SRS special function on the VDS diagnostic tool, performed 'OCS zero point learning' and 'passenger detection learning', placed standard weights in sequence to complete the calibration, cleared the fault codes, and the system returned to normal.
BYD DTC AI Analysis

BYD Qin EV450: seat wiring harness worn, causing open circuit

✓
Symptom: Airbag warning light illuminates intermittently on rough roads; operates normally on smooth surfaces. DTC B1684-00 is intermittent. Diagnosis: Inspected wiring harness under the seat. Found the OCS harness retaining clip between the seat rail and body had detached. The harness had chafed against the seat frame over time, breaking internal copper strands and creating an intermittent open circuit. Repair: Repaired the damaged harness (3 wires), re-wrapped with waterproof tape, rerouted and secured the harness to a non-moving area of the seat frame so it is not pulled during seat fore/aft movement.
BYD DTC AI Analysis

BYD Qin 80 OCS control module hardware failure

✓
Symptoms: On cold start, the airbag warning light stays on. After driving for 20 minutes with the engine warm, the light occasionally goes out. DTCs B1684-00 and B1685-00 are stored (OCS sensor fault). Diagnosis: OCS module power supply and ground circuits measured normal. Substitution testing confirmed the fault: installing this vehicle's OCS module in another normal vehicle transferred the fault with it. Traced to poor thermal stability in the OCS ECU internal circuit board, causing abnormal signal processing at low temperatures. Repair: Replaced the OCS control module (integrated under the seat). Performed module matching and calibration using a diagnostic tool. Fault completely resolved.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.