AU
U025387

U025387 is a U-category network communication DTC — Seal U

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 — Seal U
  • 1Loose front compartment wiring harness connector, backed-out terminals, or poor contact: High temperatures (such as sun exposure) cause thermal expansion and contraction, forcing terminals in connectors (such as B28 and the front compartment wiring harness mating connector) to back out, oxidize, or make poor contact. This results in intermittent communication interruptions.
  • 2Electric compressor controller power supply or ground fault: Blown fuse, faulty relay, open power supply circuit, or loose ground point causes controller power loss and communication failure.
  • 3CAN network physical layer fault: Power CAN or thermal management subnetwork wiring short/open circuit, abnormal terminating resistance (normal: approx. 60Ω; abnormal: 120Ω or open circuit), electromagnetic interference causing signal distortion
  • 4Compressor controller internal 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 disconnection, 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 a current (Active) or history (History) fault. Record the freeze frame data (ambient temperature, battery temperature, vehicle speed, etc.). Clear the fault codes and road test the vehicle to verify if the fault reoccurs.
  • 2
    Low-voltage wiring harness and connector inspection: Disconnect the battery negative terminal. Check the electric compressor low-voltage wiring harness connectors (focus on the front compartment harness interface connector and right domain controller Port E). Verify pins are not backed out, bent, corroded, or burnt. Measure power pin (+B, IG) voltage (should be 12V±0.5V) and ground pin resistance (<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 termination resistance (standard: approx. 60Ω; a 120Ω reading indicates a missing termination resistor on one side, and 0Ω indicates a short circuit). In high-temperature environments, monitor the resistance value for temperature drift.
  • 4
    High-voltage insulation test: Measure the insulation resistance between the compressor high-voltage input terminal and the vehicle body ground using a 1000V megohmmeter. Standard: ≥20MΩ (some models require ≥2MΩ). Low resistance indicates internal electrical leakage in the compressor (scroll plate seal failure or motor winding insulation damage).
  • 5
    Controller software diagnosis and reflashing: Check the software versions of the VCU, integrated body controller (B2), and compressor controller. Verify they are the latest versions and check for version compatibility issues. Reflash the controllers or perform programming and matching. If necessary, perform 'non-sleep data capture' to analyze the message loss pattern.
  • 6
    Component replacement verification: Swap the compressor controller from the faulty vehicle with one from 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)

✓
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 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.