AU
B1684-00

DTC B1684-00 indicates a functional fault in the Occupant Classification System (OCS) — Atto 8

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.

The B1684-00 fault code on the Atto 8 indicates an Occupant Classification System malfunction that affects front passenger airbag deployment. Severity is medium. Common causes include physical damage to the seat pressure sensor mat from liquid or heavy objects, and loose or corroded wiring harness connections. Visit an authorised dealer for repair. Typical repair cost data is available based on four recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Physical damage or aging of the front passenger seat OCS pressure sensor mat (common causes include water ingress, liquid seepage, prolonged pressure from heavy objects, or pet scratches breaking the sensor mat)
  • 2Loose, oxidized, poor contact, or backed-out pins 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 vehicle accident repairs, seat removal/installation, or replacement, or an interrupted calibration process causing 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 SRS system fault codes, confirm B1684-00 is an active fault, record freeze frame data, and check for related fault codes such as B1685 (OCS sensor fault).
  • 2
    Check the OCS module connector under the front passenger seat (usually a yellow waterproof plug). Confirm the locking tab fully engages. Inspect the pins for oxidation, retraction, or water ingress. Measure the power supply voltage (should be 12V ± 0.5V), ground resistance (<1Ω), and CAN-H/CAN-L line voltage (approximately 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 signs of liquid corrosion. Inspect 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 and select "OCS Calibration". Follow the prompts to sequentially complete the empty seat calibration, place a 36kg standard weight (simulating a child) for calibration, and place a 75kg standard weight (simulating an adult) for calibration. Verify each step displays "Calibration successful".
  • 5
    If calibration fails or the fault code does not clear, replace the OCS sensor assembly (for Qin series models, typically replace the entire front passenger seat cushion assembly or the OCS sensing mat module). After replacement, perform the complete calibration procedure again and write the VIN code.
  • 6
    Reinstall all components, clear the fault code, and perform an SRS system self-check cycle (ignition switch ON-OFF three times). Confirm B1684-00 does not return and the instrument cluster airbag warning light turns off. Use a diagnostic tool to read the OCS live data stream and 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.