AU
B2AB9-97

DTC B2AB9-97 indicates an internal fault or performance abnormality in the passenger seat Occupant Classification System (OCS) sensor, not "air conditioning load too high" as stated in the original information — Atto 3

Thermal Management System

DTC B2AB9-97 indicates an internal fault or performance abnormality in the passenger seat Occupant Classification System (OCS) sensor, not "air conditioning load too high" as stated in the original information.

Integrated into the front passenger seat cushion, the sensor uses pressure-sensitive elements to detect seat occupancy and passenger weight range.

It outputs an analogue voltage signal to the airbag control unit (SRS ECU) (normal range 2.5V-3.5V, varying with weight).

The ECU sets this code upon detecting a fixed sensor signal voltage, a voltage outside the valid range, or an open or short circuit exceeding the threshold.

This fault forcibly disables the front passenger airbag (the instrument cluster displays "PASS AIR BAG OFF") or prevents the system from adjusting the deployment strategy based on passenger weight during a collision.

This severely compromises passive safety system operation and requires immediate repair.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1OCS sensor unit damaged: Internal pressure-sensitive resistor or circuit board failure causes a constant output signal voltage (e.g., stuck at 0.2V or 5V) that does not change with seat load.
  • 2Wiring harness connection fault: poor contact at the yellow dedicated connector under the seat, terminal pin back-out, broken locking tab, or seat slide rail adjustment pulling the wiring harness and causing an intermittent open circuit.
  • 3Liquid ingress damage: Rainwater, beverages, or wet clothing seeping into the seat cushion causes sensor circuit board oxidation, short circuits, or degraded insulation performance.
  • 4Software and calibration faults: SRS control unit software version mismatch, calibration data loss due to OTA upgrade, or incomplete weight learning procedure after replacing the seat or sensor.
  • 5System configuration error: After replacing the seat assembly during accident repairs, the OCS sensor ID code stored in the SRS ECU does not match the new hardware, or configuration parameters are not written.
What To Do
  • 1
    Diagnostic scan: Use BYD VDS or ED400 to enter the SRS system and read the freeze frame data for DTC B2AB9-97. Check if the 'OCS sensor voltage' in the data stream varies with pressure within the 2.5V-3.5V range. A constant 0V or 5V indicates an open or short circuit in the sensor or wiring.
  • 2
    Basic circuit inspection: Inspect the yellow 2-pin or 3-pin connector under the front passenger seat (usually located beneath the front of the seat) for looseness, water ingress, or corrosion. Measure the connector power supply terminal voltage to ground; it should be 12V (ignition ON). Measure the ground terminal resistance to ground; it should be less than 1Ω.
  • 3
    Dynamic wiring harness test: Adjust the seat forward and backward and check if the wiring harness interferes with the seat rail. Gently wiggle the wiring harness connector while monitoring the diagnostic tool data stream voltage for fluctuations to identify an intermittent connection.
  • 4
    Sensor body inspection: Remove the front passenger seat cushion (remove the seat retaining bolts and flip the seat) to locate the OCS sensor integrated into the foam. Measure the sensor resistance. The normal range is about 1.5-3 kΩ (refer to the repair manual for exact specifications). If the resistance is infinite or zero, or if water stains or oxidation are visible, replace the sensor or the seat cushion assembly.
  • 5
    Replacement and protection: After replacing the sensor, verify the seat foam is dry. If necessary, install a waterproof breathable membrane to prevent recurring water ingress. Verify the correct installation position of the new sensor and seat cushion without folding or compression.
  • 6
    System calibration: Use the diagnostic tool to perform 'OCS Zero Point Calibration' (unloaded state) and 'Weight Learning' (loaded state). On some models, manually enter the barcode ID on the sensor label or perform automatic configuration. Clear the fault code after calibration.
  • 7
    Function verification: Test the following three conditions separately: unoccupied seat; place an object under 5 kg on the seat (system should identify as a child, airbag OFF); seat an adult (system should identify as an adult, airbag ON). Confirm the instrument cluster indicator logic is correct and the fault lamp turns off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Plus DM-i (2021) - Sensor unit damaged

✓
**Symptoms**: After starting the vehicle, the airbag warning light on the dashboard stays on. The passenger airbag displays "OFF" regardless of front passenger seat occupancy. **Diagnosis**: Connected VDS2000 and read the SRS data stream. The passenger-side OCS sensor output voltage remained constant at 0.2V (normal: varies between 2.5V–3.5V with pressure). Removed and inspected the yellow connector beneath the seat—no looseness or water ingress. Measured harness supply voltage (12V) and ground—both normal. Determined the sensor's internal pressure-sensitive element was damaged. **Solution**: Replaced the front passenger seat occupant sensor assembly (integrated in the seat cushion foam). Performed "OCS zero calibration" and "weight learning" after replacement (path: SRS system → Special functions → Seat occupant sensor calibration). Cleared fault codes after calibration. System returned to normal occupancy recognition.
Original source ↗
BYD DTC AI Analysis

BYD Song Pro DM (2022) - Poor Contact in Wiring Harness Connector

✓
Symptoms: After interior cleaning, the airbag warning light came on intermittently and sometimes went out on bumpy roads. DTC B2AB9-97 was intermittent. Diagnosis: Inspected the wiring harness under the front passenger seat. Found the OCS sensor harness plug interfered with the seat rail during fore-aft adjustment. The plug locking clip was broken and the pins had poor contact. Tugging during cleaning worsened the contact. Voltage at the sensor plug dropped out when the harness was wiggled. Solution: Repaired the harness plug, replaced the damaged plug housing, re-crimped the harness terminals, and secured the harness to the seat frame with cable ties, routing it away from the rail. Cleared the DTC and tested with multiple seat adjustments; the fault did not recur.
Original source ↗
BYD DTC AI Analysis

BYD Han EV (2020) - Sensor damage from water ingress

✓
Symptoms: Airbag warning light illuminated after driving in heavy rain or through deep water. DTC B2AB9-97 stored. Front passenger seat had been covered with wet clothing for an extended period. Diagnosis: Removed and inspected front passenger seat. Found water stains at OCS sensor housing seal; internal circuit board oxidised. Liquid penetrated seat fabric into sensor, causing signal drift. Measured sensor resistance out of spec (normal ~1.5–3 kΩ). Fix: Replaced front passenger seat cushion assembly (OCS sensor and cushion are highly integrated; separate replacement cannot guarantee sealing). Checked seat foam for moisture and replaced damaged sections. Calibrated OCS system after installation. Added waterproof breathable membrane above sensor for extra protection.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM-p (2023 model) — Software program and calibration lost

✓
Symptoms: After OTA or multimedia system update, airbag warning light suddenly lit with DTC B2AB9-97. No accident or seat removal history. Diagnosis: Scanned with latest diagnostic tool. Found SRS control unit software version incompatible with OCS sensor hardware communication protocol, or seat occupant sensor calibration data lost during upgrade. Physical wiring and sensor appearance checked normal. Software logic error. Resolution: Reflashed SRS (airbag control unit) with latest software and performed complete OCS system configuration and calibration (zero-load and loaded). Some cases also require OCS sensor firmware upgrade. Cleared fault codes after programming. System normal.
Original source ↗
BYD DTC AI Analysis

BYD Seal (2022) - Configuration error after accident repair

✓
Symptoms: Front-end collision. After replacing the front bumper, airbags and other components, the airbag warning light came on at post-repair delivery. Retrieved DTC B2AB9-97: passenger airbag disabled. Diagnosis: Removed and reinstalled the passenger seat during collision repair. The sensor connector was secure, but the OCS sensor serial number or configuration parameters in the SRS control unit did not match the vehicle (possible causes: replacement seat assembly from a different batch, or incorrect data write to the control unit). The diagnostic tool displayed 'OCS not configured' or 'Configuration error'. Solution: Used the diagnostic tool to access the SRS system → Programming/Configuration, and rewrote the correct OCS sensor ID code (manually input the barcode from the sensor label or performed automatic recognition). After completing the configuration, ran the 'System Learn' or 'Calibration' procedure so the control unit recognizes the new sensor. Cleared the fault codes; the airbag warning light went out and the collision recovery system returned to normal.
Original source ↗
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.