AU
B2AB81C

This DTC indicates abnormal three-phase inverter voltage in the electric A/C compressor (electric scroll compressor) drive system — Seal U

Thermal Management System

This DTC indicates abnormal three-phase inverter voltage in the electric A/C compressor (electric scroll compressor) drive system.

Specifically, the Intelligent Power Module (IPM) driving the compressor's built-in Permanent Magnet Synchronous Motor (PMSM) detects the equivalent voltage of any U/V/W phase exceeding the safety threshold (typically >650V DC peak voltage after PWM modulation).

This fault triggers a protective compressor shutdown, disabling the A/C system.

It may also derate the vehicle thermal management system, affecting power battery cooling and heating functions.

Fundamentally, this indicates a high-voltage drive circuit or voltage sampling circuit fault.

Determine whether the condition is a true overvoltage (high-voltage bus fault) or a false overvoltage (sensor drift).

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Abnormal bus voltage surge in the high-voltage power distribution system: such as DC fast charger overvoltage output, abnormal regenerative braking energy recovery causing momentary traction battery overcharge, or a precharge/voltage divider resistor fault in the high-voltage power distribution box causing distorted voltage sampling.
  • 2Compressor controller (IPM module) voltage sampling circuit fault: Voltage divider resistor drift, damaged operational amplifier, or failed A/D converter chip causes the controller to falsely detect high phase voltage.
  • 3Compressor three-phase high-voltage wiring harness insulation damage: Harness wear causes high voltage to leak into the low-voltage signal circuit, or shielding failure introduces electromagnetic interference, causing erratic voltage sampling values.
  • 4Compressor motor insulation deterioration: Insulation resistance between the motor winding and housing drops (<20MΩ), generating abnormal back EMF or leakage current that the controller misjudges as overvoltage.
  • 5Software calibration or control strategy fault: Communication delay between the BMS and A/C controller causes an abnormal voltage command, or the overvoltage threshold (OVP) calibration in the compressor drive software is too sensitive.
What To Do
  • 1
    Read freeze frame data: Use the BYD VDS2000/3000 diagnostic tool to read the freeze frame at the moment the fault occurred. Record the high-voltage bus voltage (HV Voltage), phase current (Phase Current), compressor speed (RPM), and IGBT temperature to determine whether the fault is a steady-state overvoltage or a transient spike.
  • 2
    High-voltage system voltage verification: In Ready mode, use a CAT III 1000V multimeter to measure the total traction battery voltage and compare it with the diagnostic tool reading; the deviation must be <5V. Check the fast charging port CC/CP signals and BMS data stream to rule out charging system overvoltage.
  • 3
    Compressor insulation resistance test: Disconnect the compressor high-voltage manual service disconnect (MSD). Wait 5 minutes for the capacitors to discharge. Use a 1000V megohmmeter to measure the insulation resistance between the three-phase lines (U/V/W) and the compressor housing. Standard value: >500MΩ (Qin EV). If <20MΩ, replace the compressor.
  • 4
    Low-voltage signal circuit check: Check the 5V reference voltage (Vcc), voltage sampling signal (Vout), and ground (GND) pins on the compressor controller low-voltage connector (usually 12-pin). Normal value is 5V ± 0.25V. Verify the wiring harness shield ground is secure.
  • 5
    Swap verification test: Swap the electric compressor assembly with one from the same model (or swap only the compressor controller, if detachable). Run the air conditioning at full load for 30 minutes. Observe if the fault transfers to isolate the faulty component.
  • 6
    Software update and matching: If hardware is normal, update the Air Conditioning Controller (ACU) and Battery Management System (BMS) software to the latest version, then perform compressor initialization matching (some models require 'compressor phase learning').
  • 7
    System reset and verification: Clear the fault code, perform high- and low-voltage charge and discharge tests (fast charging + high-power cooling), continuously monitor the phase voltage data stream, and confirm no abnormalities before releasing the vehicle.
Real-World Cases
BYD DTC AI Analysis

Qin EV Air Conditioning Failure After Fast Charging

✓
After charging with a 120kW fast charger from a particular brand, the dashboard displayed "Air Conditioning System Fault". DTC B2AB81C. Freeze frame showed the bus voltage reached 738V when the fault occurred (normal maximum 680V). Found the fast charger output voltage had drifted. BMS overvoltage protection activated late, causing the compressor controller to trigger phase voltage protection. Fault cleared after switching to a different fast charger. Advised the customer to avoid this brand of charging station.
BYD DTC AI Analysis

Compressor controller sampling circuit fault

✓
Air conditioning suddenly stopped working while driving; no collision history. Scanner read DTC B2AB81C. Live data showed the compressor phase voltage jumping between 0–800 V while the actual bus voltage held stable at 650 V. Disassembled the compressor controller (IPM) and found a cold solder joint on voltage divider resistor R15 (100 kΩ) causing sampling drift. Resoldered the resistor and applied conformal coating. Fault resolved.
BYD DTC AI Analysis

Three-phase harness insulation failure from wear.

✓
After flood damage repairs (flood depth ~40 cm), the air conditioning intermittently failed to cool, setting DTC B2AB81C. Inspection found the compressor three-phase high-voltage harness chafed at the front cabin firewall penetration; damaged insulation caused high-voltage leakage to chassis (insulation resistance only 2 MΩ). Replaced the compressor harness assembly and resealed to IP67. The fault has not recurred.
BYD DTC AI Analysis

Improper software calibration caused false positive

✓
2019 Qin EV triggered DTC B2AB81C, commonly during rapid acceleration followed by immediate AC activation. Technical bulletins indicate early software used an overly conservative phase voltage OVP threshold of 620V. The updated software raises this to 680V and optimizes voltage filtering. Reflashed the ACU to V2.3.5; fault eliminated.
BYD DTC AI Analysis

Compressor motor winding insulation deterioration

✓
A Qin EV taxi with 180,000 km exhibited gradually deteriorating air conditioning cooling performance until complete failure, logging DTC B2AB81C. Insulation resistance between the compressor three-phase lines and housing measured 15 MΩ, 8 MΩ, and 0.5 MΩ respectively (severely uneven). Disassembly revealed aged, flaking motor winding insulation varnish with localized breakdown to the housing. Replaced the electric compressor assembly and recovered the old unit for motor teardown analysis, confirming insulation deterioration from prolonged high-load operation.
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.