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 8

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 8
  • 1Loose compressor three-phase high-voltage harness connector (U/V/W phases), or backed-out, oxidized, or burned 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) prevents complete three-phase waveform output. This failure 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, typically resulting from compressor overheating or mechanical seizure due to insufficient refrigerant.
  • 4Poor contact at the compressor low-voltage control connector (12V power supply/ground/CAN line) causes abnormal enable or PWM control signals, disrupting drive timing and triggering a 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 set a protective phase-loss fault.
What To Do
  • 1
    Use BYD VDS or X-431 to read complete fault codes and freeze frame data. Record vehicle speed, SOC, compressor speed, and three-phase current values (Iu/Iv/Iw) at the time of the fault. Confirm if the fault is current or historical, and check for related faults such as B2AB049 (current sampling circuit fault).
  • 2
    Perform the high-voltage power-down procedure (disconnect the service disconnect and wait 5 minutes). Remove the front compartment lower undertray. 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
    Disconnect the high-voltage wiring harness and use a multimeter to measure the static resistance of the compressor three-phase windings at the compressor body terminals. Normal resistance is 0.8-1.5Ω (depending on the compressor model), with a difference between phases of <10%. Infinite resistance on any phase indicates an open circuit in the winding. A three-phase imbalance >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 to the compressor housing/vehicle body ground). Normal resistance is >500MΩ. Insulation resistance <20MΩ indicates an electrical leakage risk. Inspect for wiring harness damage or internal compressor insulation failure.
  • 5
    Check the supply voltage (12 V ± 0.5 V), ground resistance (< 1 Ω), and CAN line waveforms (CAN-H 2.5–3.5 V, CAN-L 1.5–2.5 V) at the compressor low-voltage control connector (usually 4–6 pins). Measure the enable signal wire (usually 12 V or a PWM signal in the ON position).
  • 6
    Reconnect the high-voltage wiring harness. After power-on, use the diagnostic tool to perform a compressor active test (gradually increase speed to 3000rpm) while reading the real-time three-phase current values in the data stream. The three phases should normally balance (difference <10%). If any phase current remains zero or fluctuates abnormally, replace the compressor assembly or controller.
  • 7
    If the above checks are normal but the fault occurs intermittently, consult the BYD Technical Service Bulletin (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 positives resulting from overly strict sampling threshold settings.
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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.