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B1684

In BYD vehicles, DTC B1684 indicates an Occupant Classification System (OCS) fault — Atto 8

Safety System

In BYD vehicles, DTC B1684 indicates an Occupant Classification System (OCS) fault.

The OCS mounts inside the front passenger seat and consists of the OCS control module and pressure sensors.

The system detects pressure distribution and weight on the seat surface to identify the front passenger seat status (empty/child/adult).

It sends occupant presence and type signals to the airbag control unit (SRS ECU).

The SRS ECU uses these signals to determine the front passenger airbag deployment strategy (whether to deploy and deployment force) during a collision.

When the SRS ECU detects a communication loss with the OCS module, an abnormal sensor signal, or missing system calibration data, it logs DTC B1684-00, illuminates the airbag warning light, and enters fail-safe mode (typically disabling front passenger airbag deployment).

Consequently, the system may fail to protect the front passenger effectively during a collision, or it may deploy accidentally when no one occupies the seat, causing secondary injury.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Internal circuit fault or software crash in the OCS control module (located under the front passenger seat), preventing sensor signal processing or causing loss of communication with the SRS ECU.
  • 2The seat pressure sensor (bladder sensor, integrated into the seat foam) is damaged, leaking air due to aging, or has an open circuit, preventing accurate pressure change detection.
  • 3Under-seat wiring harness connector (yellow waterproof connector) is loose, oxidized, corroded by water ingress, or has poor contact, especially common after driving the vehicle through water or cleaning the interior.
  • 4OCS system calibration data lost or initial calibration not performed (e.g., failing to perform Occupant Classification Learning with a diagnostic tool after replacing the front passenger seat assembly, seat foam, or OCS module).
  • 5Short or open circuit in the CAN communication line between the SRS ECU and OCS module, or OCS module power supply/ground line fault.
What To Do
  • 1
    Connect the BYD VDS or Launch X-431 diagnostic tool and enter the SRS (airbag) system. Read and record the fault codes. Confirm B1684-00 is present and will not clear. Check for accompanying OCS-related fault codes such as B1680 and B1681.
  • 2
    Visually inspect the yellow OCS module connector under the front passenger seat. Verify the connector lock is fully engaged. Inspect the terminals for signs of oxidation, backed-out pins, or water ingress. If necessary, clean with electrical contact cleaner and apply conductive grease.
  • 3
    Measure the OCS module power supply voltage (standard value: 9-13V, ignition ON) and ground resistance (standard value: <1Ω). Check the CAN-H and CAN-L circuit voltages (standard values: CAN-H approx. 2.6-2.8V, CAN-L approx. 2.2-2.4V).
  • 4
    Perform the OCS system calibration procedure: Use the diagnostic tool to access SRS system special functions and select 'Occupant Classification Learning'. Follow the prompts to sequentially place 6kg (simulating empty seat/child), 36kg (simulating child), and 75kg (simulating adult) standard weights at the specified seat positions to complete system relearning.
  • 5
    If calibration fails or the fault persists, remove the front passenger seat. Check the seat pressure sensor bladder for damage or leaks. Measure the sensor resistance (reference value: approx. 100-300 Ω, depending on model). If abnormal, replace the seat cushion assembly (sensor not supplied separately).
  • 6
    After replacing the OCS module or seat assembly, perform the calibration procedure, clear the fault codes, and perform on-vehicle verification: verify the instrument cluster 'PASSENGER AIR BAG' indicator displays correctly with an empty seat, a light object (<5kg), and an adult seated.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Pro — OCS module internal fault causing airbag light to stay on

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Symptoms: After startup, the airbag warning light on the instrument cluster stays on. Retrieved SRS DTC B1684-00 (OCS system fault) using VDS. After clearing the code and restarting, the warning light comes back on. Passenger airbag indicator always shows 'OFF'. Diagnosis: Checked OCS connector under seat – not loose, no water ingress. Power supply to module: 12V, normal. CAN communication voltage normal. Attempted OCS calibration procedure; scan tool displayed 'Learning Failed'. Diagnosed internal processor fault in the OCS control module, unable to complete calibration. Solution: Replaced passenger seat OCS control module assembly (P/N BYD-OCSECU-01). After installation, ran calibration using standard weights (6kg, 36kg, 75kg) for three-stage learning. Cleared DTC, airbag light went out, system returned to normal.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM – Seat wiring harness wear caused intermittent B1684 fault

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Symptoms: The airbag warning light on the instrument cluster illuminates intermittently when driving on bumpy roads, and sometimes goes out on its own on smooth roads. The scan tool reads DTC B1684-00 as an intermittent fault. Diagnosis: Checked the OCS module connector – tight. Wiring continuity measured normal. However, when I manually rocked the front passenger seat, the fault code came back. Removed the seat and found the wiring harness retaining clip at the seat rail-to-body mounting point had come loose. The harness was rubbing against the rail edge during fore-aft seat movement, damaging the CAN-H wire insulation and causing an intermittent short to ground. Repair: Repaired the damaged wiring by re-soldering and insulating with heat-shrink tubing. Rerouted the harness and added an abrasion-resistant sleeve. Re-secured the retaining clip so the harness won't get pinched when the seat moves. Cleared the fault code, cycled the seat back and forth repeatedly to test. Fault gone.
Original source ↗
BYD DTC AI Analysis

Generic OBD-II definition - Mirror switch circuit open (non-OCS related)

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In some generic OBD-II databases, B1684 means "Mirror Driver/Passenger Switch Circuit Open", not an OCS fault. This definition appears on certain non-BYD vehicles or early models. Symptoms: Complete failure of electric mirror adjustment; the switch does not respond. Diagnosis: A BCM scan retrieved code B1684. Inspecting the mirror switch connector revealed a broken wiring harness at the door hinge, creating an open circuit in the switch power supply. Solution: Repair the wiring break at the door hinge, or replace the mirror switch assembly. On BYD new energy vehicles, B1684 usually indicates an OCS system fault. However, if this code appears in the BCM, check the mirror switch circuit.
Original source ↗
Other Atto 8 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.