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) — Seal U

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 — Seal U
  • 1Resistance drift in the PTC heater left heat sink temperature sensor (NTC thermistor) or a signal circuit short to ground sends a false high-temperature signal to the air conditioning controller (ACU).
  • 2Heater circuit electronic water pump failure, jammed impeller, or abnormal PWM control causes insufficient coolant flow, failing to promptly dissipate PTC operating heat.
  • 3Localized breakdown, short circuit, or aging of the ceramic heating element inside the PTC heater causes abnormal heat concentration, exceeding the designed heat exchange capacity of the heat sink.
  • 4Dust buildup on the heat sink surface, coolant crystallization, or an air lock or blockage in the circuit causes a sharp drop in heat exchange efficiency.
  • 5Software bug in the Thermal Management System (TMS) controller or air conditioning controller causes PTC power regulation failure, resulting in continuous full-power heating that fails to step down.
What To Do
  • 1
    High-voltage safety protection: Wear 1000V-rated 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 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 low-voltage plug of the PTC assembly left temperature sensor (usually located on the left side of the PTC housing; harness colour code may be yellow/black or white/black) for backed-out pins, oxidation, water ingress, or corrosion. Measure continuity and insulation between the plug and the ACU.
  • 4
    Sensor performance test: Remove the left temperature sensor and measure its resistance using a multimeter (standard value at 25°C: approx. 10kΩ ±5%). Compare the reading against the NTC temperature-resistance curve table to confirm no sensor drift. Check the sensor installation torque and the condition of the thermal conductive silicone grease.
  • 5
    Cooling system diagnosis: Check if the expansion tank coolant level is between MAX and MIN. Check the heater circuit electronic water pump for operating sound and pipe vibration. Use the diagnostic tool to perform a water pump active test and observe if 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 setting) to measure the insulation resistance between the PTC high-voltage positive and negative terminals and the housing. The standard value is ≥500 MΩ. A reading of <20 MΩ indicates an internal short circuit. Use a diagnostic tool to operate the PTC at low power. Use an infrared thermal imager to verify uniform temperature distribution on the left heat sink.
  • 7
    Cooling circuit maintenance: If coolant is cloudy or discolored, recover old coolant. Flush heater circuit with BYD special cleaning agent. Clear air locks, refill with LEC-II low-conductivity coolant, and perform 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; verify the rate is <2°C/s with no abnormal spikes to confirm the fault is 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 Seal U 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]