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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 3

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
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

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

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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

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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.

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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

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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 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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.