AU
B123917

The DTC definition varies by model year; rely on the diagnostic tool's actual description — Atto 3

Thermal Management System

The DTC definition varies by model year; rely on the diagnostic tool's actual description.

For 2022–2024 BYD Seal, Han DM-i, Tang DM-p, and similar models, technical bulletin TSB-BYD-2023-B12-017 defines B123917 as "left seat horizontal adjustment motor position feedback circuit open/short to ground" within the Body control system.

This indicates the voltage signal from the Hall-effect position sensor (built into the driver's seat horizontal adjustment motor) to the Seat Control Unit (SCU) exceeds the calibrated upper limit (normal: 0.5–4.5V; fault condition: >4.8V or short to ground detected).

A failed sensor, a signal wire shorted to power, or a faulty SCU internal sampling circuit causes this condition.

Symptoms include seat memory function failure, abnormal automatic return, or interrupted adjustment.

However, early documentation for the 2017–2019 Qin EV300 and Qin Pro DM incorrectly labels this code as a thermal management system fault: "low-side voltage overvoltage" (abnormal A/C low-pressure sensor signal).

Distinguish between these definitions during diagnosis.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1A damaged internal Hall position sensor in the driver's seat horizontal adjustment motor or a misaligned magnetic ring causes an abnormally high feedback signal voltage (>4.8V) or an open circuit.
  • 2Relative movement between the seat wiring harness and the seat rail causes wear, shorting the position feedback signal wire (GJ51-18#) to the +12V power supply wire.
  • 3Seat Control Unit (SCU) internal signal sampling circuit fault, such as an abnormal pull-up resistor causing excessively high reference voltage.
  • 4Backed out pins, oxidation, water ingress, or poor contact at harness connector (GJ51/GJ52), causing an intermittent open circuit or abnormal impedance in the signal line.
  • 5Mechanical binding in the seat adjustment mechanism stalls the motor, causing an abnormal current rise that indirectly affects position feedback signal stability.
What To Do
  • 1
    Confirm DTC definition: Use the VDS2000 diagnostic tool to read the complete fault description and freeze frame data. If the display shows 'left seat horizontal adjustment motor position feedback circuit open/short to ground' (corresponding to TSB-BYD-2023-B12-017), perform the seat system inspection procedure; if the display shows an air conditioning system-related fault, switch to thermal management system troubleshooting.
  • 2
    Visual and physical inspection: Remove the driver's seat, keeping the wiring harness connected. Inspect the wiring harness under the seat for wear, focusing on the seat tracks and slide rail retaining clips. Check for damaged insulation, backed-out pins, signs of water ingress, or a wiring harness crushed by the seat frame.
  • 3
    Static circuit measurement: Disconnect the seat control module connector, turn the key to ON, and use a multimeter to measure the voltage to ground of the horizontal adjustment motor position feedback signal wire (GJ51 connector, pin 18). Normal voltage is 0.5-4.5V (varies with seat position). If the voltage is >4.8V or equals battery voltage, this indicates a short to power; if the voltage is 0, this indicates a possible open circuit.
  • 4
    Sensor body test: Remove the seat horizontal adjustment motor and measure the built-in Hall sensor power supply (12V ± 0.5V), ground (<0.1Ω), and signal wire resistance. Signal wire-to-ground resistance must be 1.5-2.5 kΩ and change linearly when manually rotating the motor. If the resistance is infinite, shorted, or changes discontinuously, replace the motor assembly.
  • 5
    Harness continuity and insulation test: Measure harness continuity between the seat control module and the motor (must be <1Ω). Measure the insulation resistance of the signal wire to the +12V power wire and to ground (must be >10MΩ). Inspect for harness fatigue points in the seat rail fore-and-aft sliding area (travel approximately 200mm). Reinforce protection using corrugated conduit.
  • 6
    Module cross-check: If the wiring harness and sensor are normal, swap the driver and front passenger seat control modules. Observe whether the fault code transfers with the module to confirm an SCU internal ADC sampling circuit fault.
  • 7
    Repair and Verification: Repair or replace the damaged wiring harness/sensor. Re-secure the wiring harness to prevent interference with moving parts. Apply conductive grease and waterproof sealant to the connector pins. Clear the fault code. Adjust the seat forward and backward at least 10 times. Confirm the fault code does not return and the memory function operates normally.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Han EV seat memory module communication fault (Reference case: B123D14)

✓
Symptoms: Driver's seat memory function inoperative; seat does not automatically adjust to stored positions; dashboard intermittently shows 'Check Seat System' warning. Diagnosis: Scanned using VDS2000. Seat memory control module stored DTC B123D14 (Left seat horizontal motor signal open circuit). Inspected wiring harness under seat and found the motor connector pin had backed out. Measured motor resistance at 2.8Ω (normal range 2.5–3.5Ω). Continuity test revealed intermittent open circuit between GJ51 connector pin 18 and body control module. Resolution: Reseated the seat horizontal motor connector pin, applied conductive paste to the harness connector, and cleared the DTCs. Seat memory function returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM passenger seat control failure (Reference case: B124117)

✓
Symptoms: All adjustment functions of the front passenger power seat failed, including fore-aft sliding, backrest angle and height adjustment, with no response from any controls. The heating function operated normally. Diagnosis: Connected the diagnostic tool and retrieved DTC B124117 (front passenger seat control module power circuit open). Consulted the wiring diagram and checked fuse F4/15 (15A) – normal. Removed the front passenger seat for inspection and found the seat control module power wiring harness worn through and broken at the seat rail clip, causing loss of +12V supply. Module communication was normal with CAN line voltages at 2.5V/2.3V. Resolution: Repaired the damaged power wiring harness, rewrapped it with abrasion-resistant corrugated tubing, and repositioned the harness clip to prevent interference with moving components.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song Pro seat position sensor signal fault (Reference case: B123A17)

✓
Symptoms: After starting the vehicle, the seat auto-return function malfunctioned. While driving, the seat occasionally moved by itself and the airbag system warning light illuminated intermittently. Diagnosis: The scan tool showed two related DTCs: B123A17 (seat position sensor circuit voltage above threshold) and B0099 (seat occupancy recognition sensor fault). Removed the driver's seat and found the seat track position sensor (Hall effect type) connector oxidized. The sensor supply voltage measured only 8.2V (standard 12V) and the signal wire-to-ground resistance fluctuated between 0.5–50 Ω. Water ingress under the seat had corroded the sensor. Repair: Replaced the seat track position sensor assembly, removed water from the seat foam, dried the wiring harness, and applied waterproof sealant to the sensor connector.
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. Sources: [1]