AU
B123698

DTC B123698 indicates a range/performance fault in the position sensor circuit of the driver-side (left) Temperature Blend Door Actuator — Atto 8

Thermal Management System

DTC B123698 indicates a range/performance fault in the position sensor circuit of the driver-side (left) Temperature Blend Door Actuator.

This actuator sits inside the HVAC assembly and drives the blend door to adjust the hot/cold air mix ratio at the left air outlet (0% = full cold, 100% = full hot).

The HVAC controller sets this DTC when it detects via the LIN bus or analog signal that the deviation between the actual actuator position and the target command exceeds the threshold (typically >10%), or the position sensor feedback voltage remains outside the normal range (0.5-4.5V) for a specified duration.

This fault causes left-side temperature adjustment failure, locks the temperature at a single setting, produces abnormal gear slipping noise, or forces the HVAC system into protection mode.

Note: Early service data may incorrectly label this as a coolant temperature fault.

Current technical definitions and repair practices confirm a blend door actuator control circuit fault.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Mechanical wear, breakage, or binding of the actuator's internal plastic gear set causes the motor to spin freely and fail to drive the flap.
  • 2The actuator built-in position sensor (potentiometer) is damaged, experiences signal drift, or has poor contact, preventing accurate position feedback.
  • 3Poor contact, oxidation, corrosion, short circuit, or open circuit in the LIN bus communication line (or signal line), causing abnormal signal transmission.
  • 4A/C controller software calibration data lost; actuator initialization learned values invalid (common after OTA update or power disconnection)
  • 5Vehicle wading or a blocked evaporator drain causes moisture ingress, corroding and short-circuiting the actuator internal circuit board.
What To Do
  • 1
    Connect the BYD VDS2000 or Launch X431 diagnostic tool. Access the air conditioning system to read the data stream. Check if the 'left temperature flap actual position' and 'target position' synchronize. Confirm if the fault code is current or history.
  • 2
    Remove the front passenger glove box or lower centre console trim panel. Visually inspect the left temperature flap actuator linkage for binding or disconnection. Inspect the electrical connector for looseness, backed-out pins, or oxidation.
  • 3
    Measure the actuator supply voltage (12V ±0.5V), ground resistance (<1Ω), and LIN line voltage (approx. 12V static, fluctuating between 7-11V during communication), or verify the position sensor signal voltage changes linearly between 0.5-4.5V. Inspect the wiring harness for short or open circuits.
  • 4
    Run the 'A/C air flap calibration/initialization' procedure (path: Special Functions → A/C System → Air Flap Position Learning). Delete the original calibration data, perform a full-travel (0-100%) scan and store the new calibration, then verify successful completion.
  • 5
    If calibration fails, the data stream shows abnormal values (such as 255 or a fixed value), or the actuator makes abnormal noise, replace the left temperature damper actuator assembly (common part numbers: HAC-8107010A/HAD-8107010) and verify the damper shaft has no deformation.
  • 6
    After replacement, re-run the air flap position learning procedure, clear the fault code, and test the temperature adjustment across the full 16-32°C range. Verify the data stream updates accordingly, the air outlet temperature responds normally, and no abnormal noise occurs.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Stuck blend door actuator caused loss of temperature control in Qin Plus DM-i

✓
2021 Qin Plus DM-i, 32,000 km. The left-side vent temperature would not adjust and constantly blew ambient air. Connected VDS2000 and read the live data: the left temperature blend door actual position read 255 (abnormal value), while the target position was normal. Disassembled the assembly and found the actuator linkage mechanism jammed with high resistance when moved manually. Measured the position sensor signal wire voltage and found no change, confirming the potentiometer had failed. Replaced the left temperature blend door actuator assembly (HAC-8107010A) and performed blend door calibration. Temperature control returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Song Pro DM actuator gear wear noise

✓
2020 BYD Song Pro DM, 58,000 km. Clicking noise from behind the dashboard followed by automatic climate control failure, temperature stuck on max heat. Read DTCs B123698 and B123700. Disassembled the dash and found the left blend door actuator's internal plastic gears worn and slipping, causing the motor to freewheel and actual position to deviate significantly from target. Replaced the actuator (HAD-8107010), applied grease, performed door position learning, cleared the fault.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Tang DM LIN bus intermittent poor contact fault

✓
2019 Tang DM, 71,000 km. Intermittent air conditioning fault light, DTC B123698 recurring every 3–5 days, 5-second delay in left-side temperature adjustment. Measured LIN bus voltage dropping to 2–3 V during fault events (normal 7–11 V). Found loose locking tab and oxidised pins on actuator connector above passenger footwell. Cleaned oxidation, applied conductive grease, and secured the connector. LIN communication restored to stable; no recurrence after two weeks of monitoring.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Han EV software calibration lost after OTA update

✓
2020 Han EV with 12,000 km on the clock. After an OTA head unit update, the left side had no warm air (winter), while the right side worked normally. Pulled DTC B123698, but the actuator showed no external damage and moved smoothly by hand. Swapped left and right actuators; the fault code transferred with the actuator, proving the hardware was sound. Used VDS2000 to run 'air door position learning/initialisation', deleted the old calibration, executed a full-stroke scan, and stored new data. Independent left-right temperature adjustment returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

e5 rideshare actuator short circuit due to water ingress

✓
2018 BYD e5 (ride-share vehicle), 120,000 km. Air conditioning stuck in full heat mode, cutting summer range by 40%. Retrieved DTCs B123698 and B123699 (circuit high input). Measured a short to ground on the position sensor signal wire. Disassembled the actuator and found the internal PCB corroded by moisture from a wet carpet. Replaced the left-hand temperature blend door actuator, cleared a blocked evaporator drain, dried out the wiring loom, and recalibrated the system. Temperature control range returned to normal.
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]