AU
B1684-00

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

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 Seal U indicates a functional fault in the Occupant Classification System (OCS), which detects if the front passenger seat is occupied by a child or adult to control airbag deployment. A medium-severity issue, it typically stems from a damaged seat pressure sensor (often from water or heavy objects) or loose wiring connections. This requires immediate repair at an authorised dealer. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Physical damage or aging of the front passenger seat OCS pressure sensor mat (commonly due to water or liquid ingress, prolonged pressure from heavy objects, or pet scratching causing the sensor 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 calibration and weight calibration after vehicle 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 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 active. Record the freeze frame data and check for accompanying fault codes such as B1685 (OCS sensor fault).
  • 2
    Check the OCS module connector (usually a yellow waterproof plug) under the front passenger seat. Verify the locking tab is fully engaged. Check the pins for oxidation, push-out, or signs of water ingress. Measure the 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: Enter SRS system special functions on the diagnostic tool, select 'OCS Calibration', and 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 (for Qin series models, replace the entire front passenger seat cushion assembly or OCS sensor mat module). After replacement, repeat the complete calibration procedure and write the VIN code.
  • 6
    Reinstall all components, clear the fault code, and perform the 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 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.

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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

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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

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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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.