AU
B2AB1-49

This DTC specifically indicates a phase-loss fault in the built-in three-phase permanent magnet synchronous motor of the electric A/C compressor (E-Compressor) — Atto 3

Thermal Management System

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

The compressor controller (Inverter) detects a severe three-phase current imbalance via Hall sensors or current sampling circuits (one phase current is zero or the difference is >30%), or detects an abnormal back-EMF waveform, determining a phase-loss condition.

This fault typically indicates poor contact at the compressor high-voltage harness, an open motor winding, a damaged controller power module (IPM), or an abnormal low-voltage control signal.

Because BYD uses an integrated thermal management system for the battery, motor, and cabin, the A/C compressor handles both cabin cooling and battery/motor coolant heat dissipation.

This fault causes A/C failure and, under high-load operating conditions, triggers battery or motor overheat protection, limiting power output (entering limp mode).

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Compressor three-phase high-voltage harness connector (U/V/W phases) is loose, oxidized, burned, or has backed-out pins, causing excessive contact resistance or an intermittent open circuit, especially on early Qin series models where the front compartment layout subjects the harness to vibration.
  • 2A damaged internal IGBT power module or drive circuit in the electric compressor controller (inverter) fails to output a complete three-phase waveform. This typically occurs after poor heat dissipation or a high-voltage surge.
  • 3Compressor motor winding open circuit (broken enameled wire) or phase-to-phase short circuit causing three-phase impedance imbalance. Insufficient refrigerant typically causes compressor overheating or mechanical seizure, leading to this fault.
  • 4Poor contact at the compressor low-voltage control connector (12V power supply/ground/CAN line) causes an abnormal enable signal or PWM control signal, leading to drive timing sequence errors and false phase-loss detection.
  • 5Vehicle high-voltage system precharge failure or high-voltage interlock circuit fault causes a sudden voltage drop during compressor startup, prompting the controller to report a protective phase-loss fault.
What To Do
  • 1
    Use a BYD VDS or X-431 to read the complete fault codes and freeze frame data. Record the vehicle speed, SOC, compressor speed, and three-phase current values (Iu/Iv/Iw) at the time of the fault. Confirm whether the fault is current or historical, and check for accompanying related faults such as B2AB049 (current sampling circuit fault).
  • 2
    Perform the high-voltage power-down procedure (disconnect the service switch and wait 5 minutes). Remove the front compartment lower shield. Visually inspect the electric compressor three-phase high-voltage wiring harness connector (usually marked U, V, W) for looseness, burn marks, backed-out terminals, water ingress, or verdigris oxidation. Specifically check 2018-2019 Qin PRO models for detached wiring harness retaining clips.
  • 3
    Use a multimeter to measure the static resistance of the compressor three-phase windings (disconnect the high-voltage wiring harness and measure at the compressor body terminals). The normal value is 0.8-1.5Ω (depending on the compressor model) with a difference of <10% between phases. Infinite resistance in any phase indicates a winding open circuit. A three-phase imbalance rate >15% indicates a possible inter-turn short circuit.
  • 4
    Use a 1000V megohmmeter to measure the insulation resistance of the compressor high-voltage wiring harness and motor (measure each of the three phases separately to the compressor housing/vehicle body ground). The normal value is >500MΩ. If the insulation resistance is <20MΩ, an electrical leakage risk exists. Inspect for wiring harness damage or internal compressor insulation failure.
  • 5
    Check the supply voltage (12V ±0.5V), ground resistance (<1Ω), and CAN line waveforms (CAN-H 2.5-3.5V, CAN-L 1.5-2.5V) at the compressor low-voltage control connector (usually pins 4-6). Measure the enable signal wire (usually 12V or a PWM signal in the ON position).
  • 6
    Reconnect the high-voltage wiring harness. Power on the vehicle and use the diagnostic tool to perform a compressor active test (gradually increase speed to 3000rpm). Simultaneously read the real-time three-phase current values in the data stream. The three phases should balance (difference <10%). If any phase current remains at zero or fluctuates abnormally, replace the compressor assembly or controller.
  • 7
    If all above checks are normal but the fault is intermittent, consult BYD Technical Service Bulletins (TSB) and update the compressor controller software on 2018-2019 Qin PRO DM/EV models to the latest version (e.g., V3.2.1+) to eliminate software false alarms triggered by overly strict sampling thresholds.
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM compressor three-phase wiring harness connector burned, causing intermittent phase loss

✓
While driving, the air conditioning intermittently stopped cooling. DTC B2AB1-49 appeared intermittently; freeze frame showed W-phase current at 0A during the fault. Inspection revealed obvious burn marks and blackening on the V-phase terminal of the compressor's three-phase high-voltage connector; contact resistance measured 2.3Ω (normal <0.1Ω). Early production connectors have a waterproofing design defect; rainwater leaked into the front compartment and caused electrochemical corrosion. Replaced the three-phase high-voltage harness assembly (Part No.: HA-8103010), applied conductive grease to the connector, and wrapped it with waterproof tape. Fault resolved.
BYD DTC AI Analysis

Qin EV300 compressor controller IGBT breakdown causes phase loss protection

✓
AC suddenly stopped working while driving. Dashboard displayed "Thermal Management System Fault". DTC B2AB1-49 was present and would not clear. Compressor three-phase winding resistance measured normal at 1.2Ω per phase, but power-on testing showed no current output on the W-phase. Disassembled the compressor controller (inverter) and found the W-phase IGBT module (Model: FF450R12ME4) shorted and the thermal paste dried out. Replaced the compressor controller (some vehicle models require the complete compressor assembly), reapplied thermal paste, and cleared the fault.
BYD DTC AI Analysis

Qin EV300 Compressor Motor Winding Open Circuit Replacement Case

✓
Vehicle sat for one week; AC failed to start. DTC B2AB1-49 with B2AB049 (current sampling circuit fault). Resistance at compressor three-phase terminals: U-phase open (infinite), V/W phases normal at 1.1Ω. U-phase winding wire broke inside the motor, likely from overheating due to insufficient refrigerant or vibration fatigue during prolonged low-frequency operation. Compressor is hermetically sealed and non-serviceable; replaced electric compressor assembly (PN: BC-8103020). Evacuated and recharged system with 450g R134a refrigerant. Normal operation restored.
BYD DTC AI Analysis

Case: B2AB1-49 fault code masking after software false positive

✓
The Qin Pro EV intermittently set DTC B2AB1-49, though AC cooling remained normal. Data stream showed balanced three-phase current (Iu=8.5A, Iv=8.3A, Iw=8.4A). Per BYD TSB-2020-032, certain 2018-2019 vehicles have compressor controller software V2.8.X with sampling thresholds set too strict, causing false phase-loss detection during voltage fluctuations. Fix: Upgraded compressor controller software to V3.2.1 using VDS and cleared fault codes. No recurrence after one month. This was a software false positive, not a hardware failure.
BYD DTC AI Analysis

Phase loss fault caused by loose high-voltage interlock loop connection

✓
Qin PRO DM air conditioning suddenly cuts out during hard acceleration or on rough roads. Fault codes B2AB1-49 and P1A3900 (high voltage interlock fault) stored. Inspection found the high voltage harness interlock pin between the front compartment high voltage distribution box and the compressor had backed out, causing the interlock signal to drop out intermittently while driving. The compressor controller detected overcurrent or phase loss the instant the high voltage relay opened. Fix: Repaired the high voltage harness interlock pin, re-crimped the terminal, and ensured full insertion with an audible click. Fault resolved.
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.