AU
B2AB5-73

DTC B2AB5-73 indicates the electric air conditioning compressor (E-Compressor) failed its start-up self-check — Seal U

Thermal Management System

DTC B2AB5-73 indicates the electric air conditioning compressor (E-Compressor) failed its start-up self-check.

The compressor controller (Inverter) generates this fault if it receives a start command from the VCU (Vehicle Control Unit) but fails to establish normal speed feedback or a normal high-voltage DC bus within the specified time (usually 3-5 seconds).

Technical causes include abnormalities in the high-voltage interlock (HVIL) circuit integrity, DC bus pre-charge logic, compressor motor phase current detection, or the LIN bus communication handshake protocol.

In the BYD thermal management system, this compressor handles both cabin air conditioning and battery pack liquid cooling/heating functions.

When this fault triggers, the system inhibits compressor operation.

This inhibition can cause battery thermal management failure, subsequently triggering power limitation (reduced-power driving) or preventing high-voltage power-up.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1High-voltage interlock loop (HVIL) pin backed out or corroded: Poor contact at the compressor high-voltage connector interlock pin causes the vehicle control unit to detect an incomplete high-voltage circuit and prevent compressor startup.
  • 2Compressor controller internal IGBT or drive circuit fault: A damaged power module on the controller PCB cannot output three-phase AC to drive the scroll plate, resulting in high-voltage input but no current output.
  • 3LIN bus communication fault: Short circuit, open circuit, or signal interference in the LIN wiring harness between the compressor controller and the air conditioning controller (AC ECU) prevents speed command transmission or causes feedback signal loss.
  • 4Compressor mechanical seizure or insulation failure: A seized scroll plate causes excessive startup current and triggers protection, or motor winding insulation resistance below 2MΩ triggers leakage protection.
  • 5Abnormal refrigerant pressure protection: When high-pressure side pressure exceeds 3.2 MPa or low-pressure side pressure drops below 0.18 MPa, the pressure switch cuts off the start signal to prevent compressor damage from pressurized start-up.
What To Do
  • 1
    Read the full DTC and freeze frame data using the diagnostic tool: Use VDS or ED400 to read the data streams at the time of the fault, including ambient temperature, battery SOC, compressor target speed, actual speed, high-voltage bus voltage, and compressor current, to confirm whether the fault is intermittent.
  • 2
    High-voltage interlock circuit inspection: Disconnect the compressor high-voltage connector. Measure continuity between the HVIL circuit pins (< 1 Ω). Check the connector for backed-out pins, burn marks, or signs of water ingress. Clean or replace the wiring harness if necessary.
  • 3
    High-voltage power supply and pre-charge check: With the vehicle powered on, measure the voltage between the positive and negative terminals of the compressor high-voltage connector. The voltage must equal the traction battery voltage (normal range 320V-700V). Verify pre-charge is complete (no voltage fluctuation).
  • 4
    LIN bus waveform measurement: Use an oscilloscope to measure the compressor LIN line waveform. The normal level is 12V (dominant 0V, recessive 12V). Check for a short to ground or a short to power.
  • 5
    Insulation resistance test: Use a megohmmeter (1000V range) to measure the insulation resistance between the compressor high-voltage positive/negative terminals and body ground. The standard value must be greater than 20 MΩ. A reading below 2 MΩ indicates internal compressor insulation failure.
  • 6
    Refrigerant pressure and mechanical inspection: Connect a refrigerant pressure gauge to check if the high and low-pressure side pressures are within the normal range (static pressure approximately 0.6-0.8 MPa). Manually rotate the compressor clutch (if equipped) or check if the scroll plate is seized.
  • 7
    Component replacement verification: If the above checks are normal, replace the compressor controller (if replaceable separately) or the entire electric compressor assembly, then evacuate and recharge the refrigerant (standard amount: 450-550g R410a).
Real-World Cases
BYD DTC AI Analysis

Worn compressor LIN harness causes intermittent startup failure on Qin PRO DM

✓
A 2018 Qin PRO DM with 62,000 km. The instrument cluster intermittently displayed "Thermal Management System Fault" and the air conditioning intermittently stopped cooling. VDS read code B2AB5-73; freeze frame showed compressor speed at 0 rpm when the fault occurred. High voltage supply tested normal. The LIN line showed resistance to ground occasionally dropping to 0.5 Ω. Lifted the vehicle and found the compressor LIN wiring harness rubbing against the front subframe; damaged insulation caused an intermittent short to ground. Repaired the harness and re-secured the routing. Fault resolved.
BYD DTC AI Analysis

Qin EV300: Backed-out HVIL pin prevented compressor from starting

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2017 BYD Qin EV300 with no AC cooling. Retrieved DTC B2AB5-73. Inspected the compressor high-voltage connector and found one HVIL (High Voltage Interlock Loop) shorting pin completely backed out, causing the vehicle controller to detect an incomplete high-voltage circuit. Re-crimped the pin, applied conductive paste, and fully seated the connector. Fault code cleared, compressor started normally, and HVIL resistance returned to normal (less than 1 Ω).
BYD DTC AI Analysis

Compressor controller IGBT breakdown caused Qin PRO EV startup failure

✓
2019 Qin Pro EV. Air conditioning suddenly stopped working while driving, accompanied by a 'motor overheating' warning. The diagnostic tool reported B2AB5-73 and B2AB41D (internal overcurrent). Disassembled the compressor controller and found the upper and lower bridge arms of one phase in the IGBT module had shorted, triggering immediate overcurrent protection once pre-charge completed. Replaced the compressor assembly (integrated controller and motor design), recharged the refrigerant, and fixed the fault. Poor controller cooling caused the power devices to overheat and fail.
BYD DTC AI Analysis

Insufficient refrigerant triggered low-pressure protection (B2AB5-73).

✓
2019 Qin PRO DM — AC shuts down shortly after startup. Scanned DTC B2AB5-73. Live data showed low-side pressure at 0.12MPa when the fault occurred, below the 0.18MPa protection threshold. Found micro-leak at condenser fitting; refrigerant charge at 30% of spec. Repaired leak, evacuated system and recharged with 500g R410a and refrigerant oil. Fault cleared. Analysis: Severe refrigerant loss triggered low-pressure protection immediately on compressor startup; consecutive failed start attempts set this DTC.
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.