AU
B123398

This DTC indicates the vehicle thermal management system detects the right heat sink (typically the PTC high-voltage heater module or power electronics cooling base) temperature exceeds the preset safety threshold (generally >85°C-95°C) — Atto 8

Thermal Management System

This DTC indicates the vehicle thermal management system detects the right heat sink (typically the PTC high-voltage heater module or power electronics cooling base) temperature exceeds the preset safety threshold (generally >85°C-95°C).

As an active safety protection mechanism, the vehicle limits or cuts off the PTC heating function when triggered to prevent thermal runaway from damaging high-voltage components or causing a fire risk.

The right heat sink typically corresponds to the right unit of the cabin PTC heater or a specific cooling module in the battery heating circuit.

This abnormal temperature indicates insufficient cooling capacity in the cooling circuit or a faulty heating element.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Thermal management system coolant is low or degraded, or an air lock has formed, causing a sharp drop in heat dissipation efficiency.
  • 2Electronic water pump fault, insufficient speed, or abnormal power supply causing interrupted coolant circulation or insufficient flow.
  • 3Right heat sink temperature sensor (NTC thermistor) short circuit, open circuit, or signal drift, causing a false high-temperature reading.
  • 4Breakdown, short circuit, or aging of the internal ceramic element in the PTC heater generates abnormal excessive heat that fails to dissipate in time.
  • 5Blocked, kinked, or leaking cooling lines causing restricted localized coolant circulation in the right heat sink.
What To Do
  • 1
    Use the BYD VDS2000 diagnostic tool to read the complete fault codes and freeze frame data. Check the actual 'right-side heat sink temperature' value in the data stream to determine if the overheating is genuine or a false sensor reading.
  • 2
    Visually check that the coolant level in the thermal management system expansion tank is between MIN and MAX. Inspect the coolant for a cloudy or emulsified appearance. Top up or replace with the specified antifreeze if necessary.
  • 3
    Start the vehicle and turn on the heater. Check the electric water pump for operating noise and vibration. Use the diagnostic tool to perform an active test of the water pump speed, and check the water pump supply voltage (standard 12V) and PWM signal.
  • 4
    Raise the vehicle. Check the right heat sink temperature sensor connector for oxidation or looseness. Measure the sensor resistance (approximately 10 kΩ at 25°C) and check the wiring harness insulation.
  • 5
    Disconnect the high-voltage service disconnect. Measure the PTC heater insulation resistance using a megohmmeter (should be >500 MΩ). Check if the PTC operating current exceeds the limit.
  • 6
    Perform the thermal management system bleeding procedure: Open the bleed screw and run the electric water pump to purge air from the cooling circuit. Verify no air lock remains inside the right radiator.
  • 7
    Clear the fault code. Perform a static heating test and road test. Continuously monitor the right heat sink temperature in the data stream to confirm it remains within the normal range (<80°C).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM-i right-side PTC heat sink overheating causing intermittent heater failure

✓
Model: 2021 Tang DM-i 112KM Premium. Symptoms: After switching on heating, the right-side heatsink temperature rapidly climbed above 105°C, triggering DTC B123398. The PTC entered protective shutdown, cutting off warm air. The system recovered after cooling down and restarting. Diagnosis: Data stream reading showed a 30°C temperature difference between the right and left heatsinks—clearly abnormal. Coolant level was normal, but the right PTC outlet hose felt much hotter to the touch than the left. This indicated an air lock in the right circuit. Resolution: Ran the thermal management system bleeding procedure, focusing on the right PTC circuit. After bleeding, temperatures balanced out and the fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song Pro DM: Oxidised heat sink temperature sensor connector causing false overheating warning

✓
Vehicle: 2019 Song Pro DM AWD Performance. Symptoms: In cold weather, after starting, it repeatedly threw DTC B123398. The cluster showed a thermal management system fault, though the radiator wasn't hot to the touch. Diagnosis: Live data showed the right-side radiator temperature instantly jumping to 120°C (implausible). Inspecting the sensor connector revealed severely oxidized, green pins, causing increased contact resistance that made the ECU falsely detect high temperature. Repair: Cleaned the sensor connector with electronic contact cleaner, applied conductive grease to prevent corrosion, and tightened the connector. The fault code hasn't returned.
BYD DTC AI AnalysisFrom Chinese market (translated)

Low coolant caused the right-side radiator to overheat in a BYD Qin Plus EV.

✓
Vehicle: 2021 BYD Qin Plus EV 500KM Luxury. Symptoms: Intermittent DTC B123398 during long-distance driving, accompanied by reduced heater output. Diagnosis: Coolant level in expansion tank was approximately 2 cm below MIN mark. Pressure testing revealed slight leakage at right-side PTC heater return hose joint, causing chronic coolant loss and insufficient cooling of the right-side heat exchanger. Resolution: Tightened leaking joint, topped up with dedicated antifreeze to standard level, bled the cooling system. Road-tested 50 km with no fault recurrence.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Han EV right-side radiator overheating after accident repair

✓
Model: 2020 Han EV Ultra-Long Range Edition. Background: Front-end collision; replaced front bulkhead and thermal management piping. Symptoms: DTC B123398 logged frequently after delivery, consistently appearing 10 minutes after switching on the heater. Diagnosis: Checked the accident repair records and found the right-side PTC cooling line quick-connect fitting was not fully engaged. This caused coolant to circulate only through the left side, leaving almost no coolant flowing through the right-side heat exchanger and resulting in overheating from dry burning. Resolution: Reconnected the right-side cooling line, ensuring the quick-connect fitting clicked and locked into place. Bled and refilled the system. Eliminated the fault.
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]