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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 — Atto 3

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
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.
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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.
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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.
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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.
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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.
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Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
Data confidence: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.