AU
U025387

U025387 is a U-category network communication DTC — Atto 8

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.

The U025387 fault code on the Atto 8 indicates a medium-severity CAN communication loss between the main vehicle/thermal controller and the electric air conditioning compressor controller. This can disable cabin cooling or heating and may reduce battery or motor thermal management. Common causes include loose front compartment wiring connectors, poor terminal contact, or compressor controller power/ground faults. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose front compartment wiring harness connector, backed-out terminals, or poor contact: High temperatures (such as sun exposure) cause thermal expansion and contraction, forcing connector terminals (such as B28 and the front compartment wiring harness mating connector) to back out, oxidize, or develop poor contact, resulting in intermittent communication loss.
  • 2Electric compressor controller power supply or ground fault: blown fuse, relay failure, open power supply circuit, or loose ground point, causing controller power loss and communication failure.
  • 3CAN network physical layer fault: short or open circuit in powertrain CAN or thermal management sub-network wiring; 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: Insulation resistance between the high-voltage section and ground decreases (<2MΩ), triggering a protective controller power-off or causing the BMS to disconnect high voltage, indirectly interrupting communication.
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 fault (Active) or a history fault (History). Record the 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 battery negative terminal. Check the electric compressor low-voltage wiring harness connectors (focusing on the front compartment harness interface connector and right domain controller Port E). Verify pins are not backed out, bent, corroded, or burnt. Measure voltage at the power supply pins (+B, IG) (12V±0.5V) and resistance at the ground pins (<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 value is approximately 60Ω; a 120Ω reading indicates a missing termination 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 body ground. Standard: ≥20MΩ (some models require ≥2MΩ). Low resistance indicates internal compressor leakage (scroll plate seal failure or motor winding insulation damage).
  • 5
    Controller software diagnosis and reflashing: Check the VCU, integrated body controller (B2), and compressor controller software versions. Confirm they are the latest versions and check for version compatibility issues. Reflash or program and match the controllers. If necessary, perform 'non-sleep data collection' to analyze the message loss pattern.
  • 6
    Component replacement verification: Swap the faulty vehicle's compressor controller with one from a known good vehicle, or swap the front compartment wiring harness to verify if the fault transfers. If diagnostics confirm 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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.