AU
B123098

This DTC indicates the vehicle thermal management system detects the air conditioning PTC (Positive Temperature Coefficient) heater left heat sink temperature exceeds the safety threshold (typically >85-90°C) — Atto 3

Thermal Management System

This DTC indicates the vehicle thermal management system detects the air conditioning PTC (Positive Temperature Coefficient) heater left heat sink temperature exceeds the safety threshold (typically >85-90°C).

BYD Qin series vehicles use a zoned control architecture for the PTC heater; the left heat sink corresponds to the driver-side heater core heat exchange circuit.

An abnormal rise in heat sink temperature indicates the coolant cannot effectively dissipate heat generated by the PTC.

Possible causes include coolant circulation failure, internal heat accumulation within the PTC assembly, or an abnormal temperature monitoring circuit.

Continued operation will cause thermal breakdown of the PTC ceramic elements, melt surrounding wiring harness insulation, or trigger a thermal runaway fire.

Consequently, the system triggers a Level 3 severe fault protection strategy, disconnects the PTC high-voltage power supply, and disables vehicle drive to prevent irreversible damage.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Resistance drift in the PTC heater left-side heat sink temperature sensor (NTC thermistor) or signal circuit short to ground, sending a false high-temperature signal to the air conditioning controller (ACU).
  • 2Heater circuit electric water pump fault, jammed impeller, or abnormal PWM control, causing insufficient coolant flow and preventing prompt dissipation of PTC operating heat.
  • 3Localized breakdown, short circuit, or aging of the ceramic heating element inside the PTC heater causes abnormal heat concentration that exceeds the designed heat exchange capacity of the heat sink.
  • 4Dust accumulation on the heat sink surface, coolant crystallization, or an air lock or blockage in the circuit sharply reduces heat exchange efficiency.
  • 5A software bug in the Thermal Management System (TMS) controller or A/C controller causes PTC power regulation failure, resulting in continuous full-power heating that cannot step down.
What To Do
  • 1
    High-voltage safety: Wear 1000V insulated gloves and perform the standard high-voltage power-down procedure (disconnect the service disconnect switch/negative terminal, wait at least 5 minutes, and use a multimeter to verify the system has no high voltage).
  • 2
    Diagnostic tool deep scan: Use the BYD VDS 2000/3000 to read freeze frame data. Record the left heat sink temperature, PTC operating current, coolant temperature, and water pump speed at the time of the fault. Confirm whether the fault is current or historical.
  • 3
    Wiring harness and connector inspection: Inspect the PTC assembly left temperature sensor low-voltage connector (typically located on the left side of the PTC housing; wiring harness colour code may be yellow/black or white/black) for backed-out terminals, oxidation, or water ingress corrosion. Measure continuity and insulation from the connector to the ACU.
  • 4
    Sensor performance test: Remove the left temperature sensor and measure its resistance using a multimeter (standard value at 25°C: approximately 10 kΩ ±5%). Compare the reading against the NTC temperature-resistance curve table to confirm no sensor drift. Check the sensor installation torque and thermal grease condition.
  • 5
    Cooling system diagnosis: Verify the expansion tank coolant level is between MAX and MIN. Check the heater circuit electric water pump operating sound and pipe vibration. Use the diagnostic tool to perform a water pump active test and verify the speed changes linearly with the duty cycle. Check the radiator fins for physical deformation or blockage.
  • 6
    PTC insulation and power test: Use a megohmmeter (1000V DC range) to measure the insulation resistance of the PTC high-voltage positive and negative terminals to the housing. The standard is ≥500MΩ. A reading <20MΩ indicates an internal short circuit. Use a diagnostic tool to operate the PTC at low power, and use an infrared thermal imager to verify uniform temperature distribution on the left heat sink.
  • 7
    Cooling circuit maintenance: If the coolant is cloudy or discolored, recover the old coolant. Flush the heater circuit with BYD special cleaning agent. Remove air locks, then refill with LEC-II low electrical conductivity coolant. Perform the bleeding procedure until no bubbles remain.
  • 8
    Component replacement and verification: Based on inspection results, replace the faulty PTC assembly (replace as a complete unit; do not disassemble), temperature sensor, or electric water pump. After replacement, perform an airtightness test. Clear the fault code, power on the vehicle, and test the PTC heating function. Monitor the left heat sink temperature rise rate; it must be <2°C/s with no abnormal spikes. Confirm the fault is completely resolved.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song MAX: Intermittent stalling while driving (CAN communication fault)

✓
Symptoms: The vehicle occasionally stalled while driving but would restart and run normally afterward. When the engine stalled, multiple warning lights illuminated on the instrument cluster (check engine light, ESP light, etc.). The fault was intermittent and irregular. Diagnosis: 1. An initial scan with the VDS diagnostic tool found no fault codes. 2. A deep scan of the entire vehicle system identified communication faults between the Engine Control Unit (ECU) and Transmission Control Unit (TCU). 3. Inspection of the engine bay wiring harness and ground points revealed the ECU ground wire mounting bolt had loosened, causing poor grounding and communication interruptions. Resolution: Retightened the ECU ground wire mounting bolt, inspected and secured other related wiring connections, cleared the fault codes, and test-drove the vehicle for one week. The fault did not reoccur.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song DM Vehicle Control Unit communication failure (GJK harness pin backed out)

✓
Symptoms: The vehicle would not engage the high voltage system and the dashboard displayed "Power System Fault". The diagnostic tool could not read data from the Vehicle Control Unit (VTOG), with the system reporting a communication timeout. Diagnosis: 1. Checked the VCU module power, ground and CAN line voltages; power supply was normal but the CAN signal was abnormal. 2. Traced the wiring harness to the G09 connector (located on the left side of the dashboard) and found a backed-out pin at CAN pins 14/15 on the GJK14 harness connector. 3. Checked fuse F4/9; it had not blown, ruling out a power supply fault. Resolution: Repaired the backed-out terminal at the GJK14 harness connector, re-secured the waterproof seal on the connector, and used the diagnostic tool to confirm VCU communication was restored. The vehicle now engages the high voltage system and drives normally.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin B-pillar rattle (sheet metal weld failure)

✓
Symptoms: When driving over rough roads, a creaking metallic friction noise emits from the left B-pillar area, most noticeable at 20–40 km/h. Passengers reported the noise was prominent near the seatbelt. Diagnosis: 1. Opened the left front and rear doors and road tested; the noise remained, ruling out door hinge and check strap faults. 2. Removed the left B-pillar trim and left seatbelt mounting bolts and road tested; the noise persisted, ruling out plastic trim friction. 3. Removed the driver's seat and road tested; the fault remained, ruling out seat runner noise. 4. Used a mechanic's stethoscope to check the base of the B-pillar and detected a weld failure on the rear section of the three-layer welded steel panels, with the outer steel panel rubbing against the middle panel generating the noise. Solution: Inserted red heat-resistant plastic tubing between the three B-pillar steel panels to act as a cushioning spacer, eliminating panel-to-panel friction; re-welded and reinforced the failed weld sections. Road tested for 50 km; the noise completely disappeared.
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]