AU
B2AB7-74

DTC B2AB7-74 (diagnostic tools typically display this as B2AB774) indicates the actual speed of the electric compressor or PTC heater deviates from the target speed beyond the system threshold (typically ±10%-15%), or the speed feedback signal is abnormal or missing — Seal 6 EV

Thermal Management System

DTC B2AB7-74 (diagnostic tools typically display this as B2AB774) indicates the actual speed of the electric compressor or PTC heater deviates from the target speed beyond the system threshold (typically ±10%-15%), or the speed feedback signal is abnormal or missing.

This fault indicates a thermal management system closed-loop control anomaly involving the permanent magnet synchronous motor (PMSM) field-oriented control (FOC) algorithm monitoring.

The controller triggers this fault upon detecting the speed feedback from the rotor position sensor (resolver or Hall sensor) mismatches the PWM modulation target, or when an abnormal three-phase current waveform causes a loss of synchronization.

This fault may limit air conditioning cooling/heating functions and reduce thermal management efficiency.

In extreme cases, it triggers high-voltage interlock protection and cuts off the compressor high-voltage power supply to protect the motor and controller.

5
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Internal mechanical binding or wear in the electric compressor or PTC heater, or degraded refrigerant oil, preventing the actual speed from matching the target speed.
  • 2Three-phase high-voltage wiring harness connector is burnt, loose, or has excessive contact resistance, causing drive voltage drop and speed fluctuation.
  • 3Oxidized or loose compressor/PTC controller ground point, or faulty internal 12V/5V power supply module, causing abnormal speed signal acquisition.
  • 4Coolant circulation blockage in the thermal management system (e.g., scale build-up in the PTC coolant passage or water pump failure) triggers overheat protection, causing the controller to actively limit speed.
  • 5Hardware fault in the IGBT module, filter capacitor, or resolver decoding chip on the controller internal drive board.
What To Do
  • 1
    Use VDS or a dedicated diagnostic tool to read all fault codes and data streams. Compare the difference between 'target speed' and 'actual speed'. Confirm the operating condition when the fault occurred (start-up/running/gear shifting).
  • 2
    Perform a high-voltage system insulation test, measure the insulation resistance to ground of the compressor/PTC high-voltage positive and negative terminals (standard >500MΩ), and check the integrity of the high-voltage interlock circuit.
  • 3
    Measure the three-phase high-voltage wiring harness resistance (phase resistance <0.1Ω, phase-to-phase resistance balanced). Check the connector for burning or backed-out pins. If necessary, disassemble the connector to inspect the internal spring contacts.
  • 4
    Check the controller low-voltage power supply (12V constant power and wake-up signal) and ground points. Use an oscilloscope to monitor the speed feedback signal waveform and check for electromagnetic interference.
  • 5
    For the PTC heater: Check the coolant flow and temperature difference. Remove and inspect the internal PTC coolant passages for blockages. For the electric compressor: Measure the three-phase winding resistance (0.5-2.0Ω, balanced across all three phases) and static current draw.
  • 6
    If mechanical binding occurs, replace the compressor/PTC assembly; if an electrical connection fault exists, replace the wiring harness or repair the ground connection; if the controller fails, replace the controller or the complete compressor assembly (integrated controller).
  • 7
    Perform system initialization after repair: add the specified amount of refrigerant oil (POE68) and refrigerant (R134a/R1234yf), or replace the coolant and bleed the air. Clear the fault code and perform a road test to verify.
Real-World Cases
BYD DTC AI Analysis

Water flow blockage in BYD Dolphin PTC heater causing abnormal rotational speed

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Symptoms: AC not heating. Dashboard shows "AC Thermal Management Fault." DTC B2AB774 (Speed Abnormal). Diagnosis: VDS scan found the PTC control module reporting a speed feedback anomaly; high-voltage supply 320V normal. Removed and inspected the PTC heater. Found internal coolant passages severely blocked, reducing heat exchange efficiency. The control module falsely reported a speed abnormality due to overheating protection. Resolution: Cleaned the PTC coolant lines and replaced the coolant. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Yuan Pro Compressor Ground Point Oxidation

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Symptoms: Air conditioning suddenly stopped cooling while driving. The instrument cluster displayed an 'AC Function Limited' warning and logged fault codes B2AB774 and B2AB573. Diagnosis: High-voltage supply at 355V (normal), but the compressor speed feedback signal dropped out intermittently. Oscilloscope testing revealed interference on the signal waveform; traced the fault to an oxidised, high-resistance earth point at the compressor controller. Fix: Cleaned and tightened the earth point, serviced the wiring harness connector, and cleared the fault.
Original source ↗
BYD DTC AI Analysis

BYD Song MAX DM-i heat pump compressor mechanical seizure

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Symptoms: Intermittent "Power System Fault" and "Air Conditioning Function Limited" warnings during winter heating, DTC B2AB774. Diagnosis: Heat pump controller live data showed excessive deviation between compressor target and actual speed. Circuit check normal. Three-phase winding resistance measured normal. Diagnosed internal mechanical seizure of the heat pump compressor with high resistance during cold starts. Solution: Replaced heat pump electric compressor assembly, replenished dedicated refrigerant oil and refrigerant, and cleaned the heat exchanger.
Original source ↗
BYD DTC AI Analysis

Loose connection in high-voltage wiring harness of BYD Seal 8-in-1 assembly

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Symptoms: A/C cooling degraded after 5,000 km on new vehicle; intermittent airflow. History DTC B2AB774. Diagnosis: Vehicle uses 8-in-1 integrated electric drive unit. Inspection found slight burning inside the compressor three-phase high-voltage harness connector. Increased contact resistance caused compressor speed mismatch. Cause: connector not fully seated, generating high temperatures under sustained high current. Fix: Replaced the high-voltage harness connector, seated it properly and applied conductive paste. Fault cleared.
Original source ↗
BYD DTC AI Analysis

BYD e2 electric compressor controller power supply fault

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Symptoms: Air conditioning not cooling; instrument cluster displayed "Check air conditioning system." DTCs: B2AB774 and B2ABA1C (internal low-voltage power supply fault). Diagnosis: High and low voltage supplies were normal, and three-phase winding insulation tested normal. However, the controller's internal 12V-to-5V power module had unstable output. Disassembly revealed a bulging, leaking capacitor, causing abnormal speed feedback signal processing. Solution: Replaced the electric compressor assembly (including controller), charged refrigerant and refrigeration oil, and pressure-tested for leaks.
Original source ↗
Other Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.