AU
B2AB149

This DTC indicates a phase loss in the internal three-phase permanent magnet synchronous motor of the electric A/C compressor (E-Compressor) — Seal U

Thermal Management System

This DTC indicates a phase loss in the internal three-phase permanent magnet synchronous motor of the electric A/C compressor (E-Compressor).

When the compressor controller (inverter) supplies power to the motor U/V/W three-phase windings, it detects a loss of at least one phase current or a voltage drop exceeding the threshold.

Open windings, open high-voltage harnesses, or damaged IGBT power modules typically cause this condition.

This fault prevents the compressor from generating effective torque and forces it to stop, which disables the A/C cooling and heating functions and triggers the thermal management system derating protection.

In vehicles like the BYD Qin EV, battery cooling relies on the A/C refrigerant circuit; therefore, this fault may indirectly raise the battery pack temperature and limit charge and discharge power.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Compressor high-voltage wiring harness connector burnt or loose: The three-phase high-voltage plug (usually located on top of the compressor) generates high temperatures due to long-term vibration or excessive contact resistance, causing pin ablation and recession that results in an open circuit in one phase.
  • 2Electric compressor controller (IPM module) fault: Breakdown of the upper and lower bridge arms in one or more phases of the built-in IGBT power module, or drive circuit damage, prevents the output of a complete three-phase sine wave voltage.
  • 3Compressor motor winding burnt out: Insulation breakdown, overheating, or overload causes an open circuit (infinite resistance) or inter-turn short circuit in one phase of the internal three-phase motor winding.
  • 4High-voltage power distribution box compressor fuse blown or relay stuck: A single phase of the 500V/20A fast-acting fuse in the high-voltage supply circuit blew, or burned high-voltage contactor contacts cause a three-phase power supply imbalance.
  • 5Abnormal low-voltage control signal: CAN communication fault between the thermal management system (TMS) controller and the compressor controller, or an abnormal 12V control power supply/enable signal causing the controller to falsely detect a phase loss.
What To Do
  • 1
    Diagnostic scan: Use VDS2000/3000 to read all fault codes. Check for accompanying codes B2AB2-49 (IPM fault), B2AB3-11 (compressor overcurrent), or high-voltage interlock faults. Record freeze frame data from the time of the fault (bus voltage, three-phase current values).
  • 2
    High-voltage safety check: Disconnect the low-voltage battery negative terminal, wait 5 minutes, put on insulated gloves, remove the compressor high-voltage service switch, and verify no residual voltage remains in the high-voltage system (<60V DC).
  • 3
    High-voltage wiring harness inspection: Check the compressor three-phase high-voltage connector (orange-yellow) for signs of burning, terminal back-out, or water ingress. Use a multimeter to measure the continuity of the three-phase wiring harness at the connector end (U-V, V-W, W-U); resistance must be <1Ω. Use a 1000V megohmmeter to measure the insulation resistance between the three phases and the housing; resistance must be >500MΩ.
  • 4
    Motor winding inspection: Disconnect the compressor high-voltage connector and directly measure the three-phase DC resistance at the compressor terminals (U-V, V-W, W-U). Normal resistance is 0.5-2.0Ω with a three-phase imbalance rate <5%. If any phase reads infinite resistance, the motor winding has an open circuit; replace the compressor assembly.
  • 5
    Controller function check: Restore low-voltage power supply and use the diagnostic tool to perform the compressor Active Test. Use an oscilloscope to check the three-phase voltage waveforms at the controller output terminals. The waveforms must show symmetrical three-phase sine waves. If any phase has no waveform output, the controller is faulty. Replace the compressor controller or the entire compressor assembly (BYD usually provides the complete assembly).
  • 6
    System reset verification: After repair, clear the fault code. Vacuum-fill the thermal management system and recharge the refrigerant. Perform a 30-minute air conditioning operation test, monitor the three-phase current balance (current difference between phases <10%), and confirm the fault code does not return.
Real-World Cases
BYD DTC AI Analysis

Burnt high-voltage plug on Qin EV compressor causing intermittent phase loss

✓
Intermittent AC cooling failure. Read DTC B2AB149. Inspection found visible burning and blackening on U-phase terminal of compressor high-voltage connector. Measured U-phase contact resistance at 12Ω (normal <0.1Ω). Poor contact caused arcing under high current, gradually burning the terminal. Replaced compressor high-voltage wiring harness assembly (including connector) and applied conductive paste. Issue resolved.
BYD DTC AI Analysis

Damaged electric compressor IGBT module triggered phase loss protection

✓
Total AC failure with DTCs B2AB149 and B2AB2-49. Removed and inspected the compressor controller; found the V-phase lower bridge arm IGBT in the IPM module shorted. Compressor windings measured normal. Isolated to controller power device quality defect. BYD does not offer standalone controller repair, so replaced the electric compressor assembly. System restored to normal with balanced three-phase current.
BYD DTC AI Analysis

Refrigerant leak caused compressor overheating and phase loss

✓
Owner reported AC cooling gradually weakened until complete failure. DTC B2AB149 stored. Found severe refrigerant leak (pressure <0.1MPa). Compressor ran dry for an extended period without oil or refrigerant, overheating the motor windings and damaging insulation until the V-phase winding opened. Replaced compressor, repaired condenser leak, and charged standard refrigerant (450g R410a) and refrigeration oil. Fault resolved.
BYD DTC AI Analysis

High-voltage distribution box relay contacts burned

✓
AC cut in and out. DTC B2AB149 intermittent. Compressor relay contacts inside HV distribution box severely burned, causing W-phase power to drop out. Relay coil tested normal, but contact resistance fluctuated with vibration. Replaced distribution box relay (or complete assembly). Fault code cleared and did not return; three-phase voltage balanced and stable.
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.