AU
B1684-00

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

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 — Qin Plus
  • 1Physical damage or aging of the front passenger seat OCS pressure sensor mat (common causes include water or liquid ingress, prolonged compression by heavy objects, or pet scratching resulting in sensor mat breakage)
  • 2Loose, oxidized, backed-out pins, or poor contact at the OCS control module wiring harness connector (frequent fore-and-aft seat movement or height adjustment causing connector wear)
  • 3Failure to perform OCS system zero-point and weight calibrations after vehicle accident repairs, seat removal/installation, or replacement; or calibration interruption resulting in abnormal data.
  • 4OCS control module internal circuit board fault (ECU hardware damage, capacitor leakage, or chip program 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 the SRS system fault codes. Confirm B1684-00 is an active fault, record the freeze frame data, and check for related fault codes such as B1685 (OCS sensor fault).
  • 2
    Check the OCS module connector (typically a yellow waterproof plug) under the front passenger seat. Verify the locking tab is fully engaged. Inspect the pins for oxidation, recession, or water ingress. Measure the power supply voltage (12V ± 0.5V), ground resistance (<1Ω), and CAN-H/CAN-L line voltage (approx. 2.5V).
  • 3
    Remove the front passenger seat. Visually inspect the OCS pressure sensing mat beneath the seat foam cushion for damage, creases, foreign object punctures, or liquid corrosion. Check the wiring harness between the sensing mat and the OCS ECU for breaks.
  • 4
    Perform OCS system calibration: Enter the SRS system special functions on the diagnostic tool and select "OCS Calibration". Follow the prompts to sequentially complete the empty seat calibration, calibration with a 36kg standard weight (simulating a child), and calibration with a 75kg standard weight (simulating an adult). Verify each step displays "Calibration successful".
  • 5
    If calibration fails or the fault code does not clear, replace the OCS sensor assembly (Qin series models usually require replacing the entire front passenger seat cushion assembly or the OCS sensor mat module). After replacement, perform the complete calibration procedure and write the VIN.
  • 6
    Reinstall all components, clear the fault codes, and perform the SRS system self-check cycle (ignition switch ON-OFF three times). Confirm B1684-00 does not recur and the instrument cluster airbag warning light turns off. Use a diagnostic tool to read the OCS live data stream to verify accurate empty/occupied seat 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 Qin Plus 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.