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U025387

U025387 is a U-category network communication DTC — Qin Plus

Thermal Management System

U025387 is a U-category network communication DTC.

It indicates a CAN bus communication interruption between the Vehicle Control Unit (VCU) or thermal management controller and the Electric Air Conditioning Compressor Controller (ACCM).

This fault indicates the main control unit failed to receive valid messages (such as status, speed, and fault data) from the compressor controller within the predetermined cycle (typically 100ms-500ms).

This represents a control link communication loss rather than a mechanical failure of the compressor itself.

The fault completely disables the air conditioning system (no cooling/heating).

This failure subsequently triggers battery thermal management degradation (limiting fast charge power and affecting battery life) and causes insufficient motor cooling (triggering high-temperature protection and power reduction).

Severe cases restrict EV functions or prevent the vehicle from starting.

5
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Front compartment wiring harness connector loose, terminal backed out, or poor contact: Thermal expansion and contraction in high-temperature environments (e.g., sun exposure) cause pins in connectors (such as B28 and the front compartment wiring harness inline connector) to back out, oxidize, or make poor contact, resulting in intermittent communication loss.
  • 2Electric compressor controller power supply or ground fault: blown fuse, faulty relay, open power supply circuit, or loose ground point, causing controller power loss and communication failure.
  • 3CAN network physical layer fault: power CAN or thermal management subnetwork wiring short/open circuit, abnormal terminating resistance (normally approx. 60Ω; abnormal is 120Ω or open circuit), electromagnetic interference causing signal distortion.
  • 4Internal compressor controller fault: controller software crash, hardware damage (e.g., burnt internal power supply chip), or waterproof seal failure causing internal water ingress and circuit board corrosion.
  • 5Compressor body insulation fault: High-voltage section to ground insulation resistance drops (<2MΩ), triggering a protective controller power-off or BMS high-voltage disconnect, indirectly causing communication loss.
What To Do
  • 1
    Fault Confirmation and Data Recording: Use the VDS2000/3000 diagnostic tool to read all fault codes. Confirm whether U025387 is an Active or History fault. Record freeze frame data (ambient temperature, battery temperature, vehicle speed, etc.). Clear the fault codes and road test the vehicle to confirm if the fault reoccurs.
  • 2
    Low-voltage wiring harness and connector inspection: Disconnect the negative battery terminal. Inspect the electric compressor low-voltage wiring harness connectors (focus on the front compartment harness interface and right domain controller port E). Verify the pins are not backed out, bent, corroded, or burnt. Measure the voltage at the power supply pins (+B, IG) (specification: 12V±0.5V) and the resistance at the ground pin (<1Ω).
  • 3
    CAN network physical layer measurement: Use an oscilloscope or multimeter to measure CAN-H (2.5-3.5V) and CAN-L (1.5-2.5V) voltage to ground; disconnect the battery and measure the CAN terminating resistance (standard: approx. 60Ω; a 120Ω reading indicates a missing terminating resistor on one side; 0Ω indicates a short circuit). In high-temperature environments, monitor the resistance value for temperature drift.
  • 4
    High-voltage insulation test: Use a 1000V megohmmeter to measure the insulation resistance between the compressor high-voltage input terminal and vehicle body ground. Standard value: ≥20MΩ (some models require ≥2MΩ). Low resistance indicates internal compressor leakage (scroll plate seal failure or damaged motor winding insulation).
  • 5
    Controller software diagnosis and reflashing: Check the software versions of the VCU, integrated body controller (B2), and compressor controller to confirm they are the latest versions and check for version compatibility issues. Perform controller reflashing or programming and matching. If necessary, perform 'non-sleep data collection' to analyze message loss patterns.
  • 6
    Component replacement verification: Swap the compressor controller between the faulty vehicle and a known good vehicle, or swap the front compartment wiring harness to verify if the fault transfers. If testing confirms a faulty compressor assembly, replace the assembly and add the specified POE refrigerant oil (such as SP-A2). Never mix with mineral oil.
Real-World Cases
BYD DTC AI Analysis

Qin PLUS DM-i won't start after high-temperature exposure (wiring harness connector pin backed out)

✓
Symptoms: After high-temperature exposure, the vehicle failed to start and the instrument cluster displayed all warning lights, logging 12 communication faults including U025387, U014087, and U029887. Diagnosis: VDS scanning revealed the VCU lost communication with multiple modules. Energy Network CAN resistance measured 238.5Ω when hot (abnormal), returning to normal 67.1Ω after cooling. Inspection found loose pins backing out of the front compartment harness connector; thermal expansion caused poor contact. Solution: Repaired the front compartment harness connector and secured the pins. No recurrence after two weeks of monitoring.
Original source ↗
BYD DTC AI Analysis

Tang DM EV Function Limited (Module Version Mismatch)

✓
Symptoms: EV function limited, pure electric mode unavailable, air conditioning inoperative. Diagnosis: VDS scan showed the system was at latest version, but further inspection revealed the A/C compressor controller failed to recognize integrated body controller B2, setting DTC U025387 (Lost Communication With A/C Compressor). Power and ground at the compressor low-voltage connector measured normal; insulation resistance normal. Resolution: Software version mismatch confirmed. Performed software upgrade and matching on the A/C compressor controller and integrated body controller B2. Communication restored after refresh; EV functions returned to normal.
Original source ↗
BYD DTC AI Analysis

Qin Pro DM A/C compressor insulation failure caused communication interruption

✓
Symptoms: Instrument cluster displayed 'EV Function Limited' and the air conditioning compressor would not start. Diagnosis: Retrieved DTC U025387. Checked compressor low-voltage harness supply – normal. Used a megohmmeter to measure high-voltage insulation resistance from compressor to ground; reading was only 0.5 MΩ (far below the 2 MΩ standard), indicating internal leakage. Further disassembly revealed deteriorated scroll plate seals. Refrigerant leakage degraded motor winding insulation, forcing the controller into protective sleep mode. Resolution: Replaced the electric compressor assembly and fitted upgraded scroll plate seals. Refilled POE refrigerant oil. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Qin EV: AC Suddenly Failed While Driving (CAN Network Resistance Drift)

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Symptoms: Air conditioning suddenly stopped working while driving. Instrument cluster displayed thermal management fault; DTC U025387. Diagnosis: Static measurement showed CAN resistance normal (60Ω). When simulating driving vibration and temperature changes, powertrain CAN resistance fluctuated between 60Ω–120Ω. Found internal break in shielded twisted pair between right domain controller connector E and compressor (intermittent open), causing poor contact under vibration. Solution: Replaced CAN communication harness from front compartment to compressor and repaired shielding. Fault resolved.
BYD DTC AI Analysis

Qin PLUS EV communication fault after compressor replacement (software not configured)

✓
Symptoms: Electric compressor replaced during accident repair. AC not cooling; DTC U025387 (Loss of Communication) logged. Diagnosis: New compressor power, ground, and CAN line voltage normal, but VDS cannot recognize the compressor controller. Replacement unit is a blank part or from a different batch, missing software configuration and VIN matching. Solution: Performed the 'Replace Electric Compressor' service routine using VDS, wrote vehicle configuration parameters, and ran communication matching and calibration between the compressor and VCU. After flashing, communication established and AC returned to normal.
Other Qin Plus 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.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 Qin Plus →Qin Plus 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.