AU
B123C16

This DTC indicates the electric air conditioning compressor (EAC) high-voltage power supply circuit detects an input voltage below the normal operating threshold (typically below 250V-300V, depending on the vehicle software version) — Seal U

Thermal Management System

This DTC indicates the electric air conditioning compressor (EAC) high-voltage power supply circuit detects an input voltage below the normal operating threshold (typically below 250V-300V, depending on the vehicle software version).

In BYD thermal management systems, the high-voltage battery (300-750V DC) directly powers the electric compressor, and the controller monitors the high-voltage side voltage in real time.

Upon detecting sustained undervoltage, the system limits or stops compressor operation to protect the compressor inverter module (IPM), causing air conditioning cooling/heating failure or degraded thermal management performance.

BMS power limitations or insulation faults typically accompany this fault, which essentially indicates an abnormality in the high-voltage power distribution circuit or battery pack energy management.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1High-voltage battery pack SOC is too low (<15%) or cell voltages are severely unbalanced, causing the BMS to trigger undervoltage protection and limit discharge power.
  • 2Backed-out terminal, burn damage, or excessive contact resistance at the electric compressor high-voltage wiring harness connector (usually located near the high-voltage power distribution box in the front compartment), causing a line voltage drop exceeding 20V.
  • 3Burnt contacts on the air conditioning compressor high-voltage relay/contactor inside the High-Voltage Power Distribution Unit (PDU) increase conduction resistance (normal: <10 mΩ; faulty: up to several hundred mΩ).
  • 4Aging of the electric compressor controller internal DC-Link capacitor, or a fault in the voltage sampling circuit (voltage divider resistor network), generating a false undervoltage signal.
  • 5Battery Management System (BMS) detects a High-Voltage Interlock Loop (HVIL) anomaly and falsely reports high-voltage undervoltage to protect the system.
What To Do
  • 1
    Read freeze frame data: Use VDS2000 or a dedicated BYD diagnostic tool to read the DTC freeze frame. Record the 'system total voltage', 'compressor high-voltage side voltage', 'compressor current', and 'SOC' values when the fault occurred. Calculate the voltage difference (normal voltage difference <5V).
  • 2
    Check traction battery status: Access the BMS data stream to check total voltage, minimum cell voltage, voltage difference (must be <300mV), and SOC. If SOC <20% or cell voltage <3.0V, charge the battery before retesting.
  • 3
    Measure the high-voltage interlock loop: disconnect the service disconnect, use a multimeter to measure HVIL loop continuity (must have continuity, resistance <10Ω), and check the air conditioning compressor low-voltage connector pins for oxidation.
  • 4
    Measure the high-voltage drop: Set the vehicle to READY mode. Use an insulated multimeter to measure the voltage between the battery pack main positive and negative terminals (V1) and the electric compressor input voltage (V2). If V1 - V2 > 15 V, check the high-voltage wiring harness and contactor from the high-voltage distribution box to the compressor.
  • 5
    Check the high-voltage distribution box: Open the PDU and visually inspect the A/C compressor high-voltage contactor contacts for burn marks and check for a blown fuse. Measure the closed contactor resistance (must be <10mΩ; otherwise, replace the PDU).
  • 6
    Check the compressor controller: Verify the low-voltage power supply (12V constant power, ground) and the CAN-H (2.5-3.5V) / CAN-L (1.5-2.5V) waveforms. If the high-voltage input is normal but the fault code persists, replace the electric compressor assembly or controller.
  • 7
    Repair verification: Clear the fault code, set the air conditioning to MAX, and verify the compressor speed exceeds 6000 rpm. Run the system continuously for 10 minutes. The repair is successful if the fault code does not return.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin EV300 air conditioning not cooling with DTC B123C16

✓
A 2017 Qin EV300 with 85,000 km. Customer reported no cold air from the A/C and the instrument cluster displayed "Air Conditioning System Fault". Scan tool retrieved DTCs B123C16 (High Voltage Side Undervoltage) and B123A11 (Compressor Overcurrent). Freeze frame data showed system voltage at 312V, compressor side voltage only 241V when the fault occurred — a 71V differential. Inspection found backed-out pins in the high-voltage wiring harness connector (large orange plug) between the front compartment high-voltage distribution box and the electric compressor. Expanded female terminals caused poor contact with a contact resistance of 0.8Ω. Replaced the high-voltage wiring harness assembly (including connector). Measured voltage differential returned to 2V. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro DM thermal management fault caused power restriction

✓
2018 Qin Pro DM, 120,000 km. Instrument cluster illuminated 'Power System Fault' warning light; air conditioning inoperative, EV mode unavailable. Read DTCs P1A0000 (BMS general fault) and B123C16. Data stream showed total battery pack voltage at 386V, but compressor high-voltage side voltage was 0V. Inspection found severe contact erosion (arc marks) on the contacts of the A/C compressor positive contactor inside the high-voltage distribution box, blocking high-voltage output to the compressor. Since the compressor was inoperative, the battery pack liquid cooling system could not dissipate heat effectively, and the BMS triggered overheat protection and limited power. Replaced the high-voltage distribution box assembly; fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Intermittent B123C16 fault caused by battery cell undervoltage

✓
2019 BYD Qin Pro DM. Air conditioning intermittently stopped working, returning to normal after shutting down and restarting. DTC B123C16 logged as a history code. Inspection found the 8th cell in the traction battery dropped to 2.8 V during discharge (other cells at 3.2 V), triggering BMS undervoltage protection. This caused the system bus voltage to drop instantly, and the compressor controller shut down on detecting undervoltage. Performed cell balancing on the battery pack and replaced the 8th cell module. Cell voltage differential returned to within 50 mV. Followed up for one month; no recurrence.
BYD DTC AI AnalysisFrom Chinese market (translated)

Electric compressor controller false fault

✓
2017 BYD Qin EV300. A/C intermittently not cooling, DTC B123C16 present. Measured battery pack voltage at 402V, compressor input at 401V (normal voltage drop). Diagnostic tool read 'high voltage side 68V' in the compressor data stream, clearly inconsistent with the actual measurement. Diagnosed as internal voltage sampling circuit fault in the compressor controller (open voltage divider resistor or damaged ADC chip). Replaced electric compressor assembly (including controller). After replacement, the data stream high-voltage reading matched the actual measured value, fault resolved.
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]