AU
B123917

The DTC definition varies by model year; rely on the diagnostic tool's actual description — Atto 8

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 8
  • 1A damaged Hall position sensor or misaligned magnetic ring inside the driver seat horizontal adjustment motor 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.
  • 4Harness connector (GJ51/GJ52) terminal back-out, oxidation, water ingress, or poor contact, resulting in an intermittent open circuit or abnormal impedance in the signal line.
  • 5Mechanical binding in the seat adjustment mechanism causes motor stall, resulting in an abnormal current increase that indirectly affects position feedback signal stability.
What To Do
  • 1
    Confirm fault code definition: Use the VDS2000 diagnostic tool to read the full fault description and freeze frame data. If the tool displays '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 it displays an air conditioning system-related fault, proceed to thermal management system troubleshooting.
  • 2
    Visual and physical inspection: Remove the driver seat (keep the wiring harness connected). Inspect the wiring harness under the seat, at the seat rails, and at the slide rail retaining clips for wear. Check for damaged insulation, backed-out terminals, 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 the ON position, and use a multimeter to measure the voltage to ground at the horizontal adjustment motor position feedback signal wire (connector GJ51, pin 18). Normal voltage is 0.5-4.5V (varies with seat position). Voltage >4.8V or equal to battery voltage indicates a short to power. A voltage of 0 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 1.5-2.5kΩ and change linearly when manually rotating the motor. Replace the motor assembly if the resistance is infinite, shorted, or changes discontinuously.
  • 5
    Harness continuity and insulation test: Measure harness continuity between the seat control module and the motor (should be <1Ω). Measure insulation resistance from the signal wire to the +12V power wire and to ground (should be >10MΩ). Inspect harness fatigue points in the seat track fore-and-aft sliding area (travel approximately 200mm). Install corrugated conduit to reinforce protection.
  • 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 internal SCU 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, then repeatedly adjust the seat forward and backward more than 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 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. Sources: [1]