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) — Seal U

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).

The B2AB1-49 fault code on the Seal U indicates a phase-loss fault in the electric A/C compressor motor. Severity is medium because it can stop A/C operation and may cause battery or motor overheat protection under high load. Common causes include a loose or damaged U/V/W high-voltage compressor connector or a failed inverter power module. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Compressor three-phase high-voltage harness connector (U/V/W phases) is loose, or has backed-out terminals, oxidation, or burn damage, 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 the output of 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 abnormal enable or PWM control signals, disrupting drive timing and triggering a false phase-loss diagnosis.
  • 5Vehicle high-voltage system precharge failure or high-voltage interlock circuit fault causes a sudden voltage drop during compressor startup, triggering the controller to report 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. Determine if 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 and visually inspect the electric compressor three-phase high-voltage wiring harness connector (usually marked U, V, W) for looseness, burning, terminal back-out, water ingress, or verdigris oxidation marks. Specifically check for a detached wiring harness retaining clip on 2018-2019 Qin PRO models.
  • 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 values are 0.8-1.5 Ω (depending on compressor model) with a variance 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 against the compressor housing/vehicle body ground). The normal value is >500MΩ. If the insulation resistance is <20MΩ, an electrical leakage risk exists. Inspect the wiring harness for damage or the compressor for internal insulation failure.
  • 5
    Check the compressor low-voltage control connector (usually 4-6 pins) for supply voltage (should be 12V ±0.5V), ground resistance (<1Ω), and CAN line waveform (CAN-H 2.5-3.5V, CAN-L 1.5-2.5V). 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 occurs intermittently, consult the BYD Technical Service Bulletin (TSB). Update the compressor controller software on 2018–2019 Qin PRO DM/EV models to the latest version (e.g., V3.2.1+) to prevent overly strict sampling threshold settings from triggering false software faults.
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

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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

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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

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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

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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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.