AU
U012E87

This DTC indicates a data communication interruption between the air conditioning controller (AC ECU) and the electronic fan controller (typically the integrated control module for the condenser fan or radiator fan) — Atto 3

Thermal Management System

This DTC indicates a data communication interruption between the air conditioning controller (AC ECU) and the electronic fan controller (typically the integrated control module for the condenser fan or radiator fan).

In the BYD thermal management system architecture, the AC ECU sends fan speed control commands (based on A/C pressure and coolant temperature) to the electronic fan via the CAN or LIN bus, and receives fan operating status, speed feedback, and fault information.

If the communication signal drops continuously for more than the set time threshold (typically 500ms-1s), the system records DTC U012E87.

This fault prevents the fan from adjusting speed on demand, potentially reducing A/C cooling efficiency and causing insufficient heat dissipation for the high-voltage system (motor, battery, and power electronics).

In extreme cases, the system triggers motor over-temperature protection or limits power output; however, under most operating conditions, the vehicle remains drivable for a short time.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Loose electric fan wiring harness connector, backed-out terminals, oxidation, or water ingress corrosion, causing poor communication signal contact or an intermittent open circuit.
  • 2Electric fan controller internal PCB damage, burnt power supply chip, CAN transceiver failure, or software crash.
  • 3Open or short circuit in the CAN/LIN communication line between the air conditioning controller and the fan (including short to ground or short to power)
  • 4Electric fan power supply or ground circuit fault (e.g., blown 30A/40A fuse, relay fault, loose or corroded ground bolt), preventing normal controller operation.
  • 5Air conditioning controller internal communication module fault or software version defect causes communication handshake failure.
What To Do
  • 1
    Use the VDS2000/3000 diagnostic tool to read all fault codes and freeze frame data. Confirm if U012E87 is a current fault and check for related power supply (e.g., B12C series) or communication (e.g., U01 series) fault codes. Record parameters such as vehicle speed and temperature at the time the fault occurred.
  • 2
    Perform a visual inspection: Open the hood and check the electric fan wiring harness connector (usually located above or on the side of the fan assembly) for looseness, water ingress, mud, sand, or obvious signs of corrosion; check if the related high-current fuse in the fuse box (usually 30A-40A) is blown.
  • 3
    Power supply and ground check: Disconnect the fan connector and turn the key to the ON position. Measure the voltage between the power supply terminal and ground (must equal 12V battery voltage, maximum voltage drop 0.5V). Measure the resistance between the ground terminal and ground (must be less than 1Ω) to confirm the fan controller has a normal operating power supply.
  • 4
    Communication line inspection: Use an oscilloscope or multimeter to measure the CAN_H (approx. 2.6-3.5V) and CAN_L (approx. 1.5-2.4V) line voltages, or the LIN line voltage (approx. 7-11V). Check the lines for open or short circuits. Measure the CAN bus terminal resistance with the power off (should be approx. 60Ω, or 120Ω for a single terminal resistor).
  • 5
    Component swap verification: If conditions allow, swap the electronic fan assembly from the faulty vehicle with one from a known good vehicle and observe if the fault transfers. Alternatively, after attempting to clear the fault code, use the diagnostic tool to perform the 'Electronic Fan Active Test' and verify normal fan control.
  • 6
    Software update and replacement: Check the software versions of the air conditioning controller and the electronic fan controller. Reflash the software if a newer update is available (especially for communication logic optimization). Replace the electronic fan assembly if the fan controller hardware is damaged (the fan and controller typically come as an integrated unit). Repair and waterproof any damaged wiring harnesses.
Real-World Cases
BYD DTC AI Analysis

Water ingress corroded the Qin EV cooling fan connector, causing intermittent communication loss.

✓
A 2019 Qin EV with 30,000 km had an instrument cluster that intermittently displayed "Thermal Management System Fault", with inconsistent air conditioning cooling. DTC U012E87 (intermittent active fault) was stored. Inspection of the electric fan harness connector revealed obvious water stains and green copper oxide inside. Traced the leak to a deteriorated left front fender seal allowing rainwater to drip directly onto the connector. Repair procedure: Cleaned the male and female terminals with dedicated contact cleaner, applied conductive anti-corrosion paste, replaced the harness waterproof seal, rewrapped with waterproof tape, and repaired the body seal. Road tested for one week with no fault recurrence.
BYD DTC AI Analysis

Internal CAN transceiver chip in cooling fan control module burned out.

✓
Vehicle displayed a motor overheating warning after high-speed driving; air conditioning did not cool. The diagnostic tool showed active DTC U012E87 that would not clear. Fan supply voltage measured 12V (normal) and ground resistance measured 0.2Ω (normal), but CAN_H and CAN_L voltage to ground both read 0V (normal is approximately 2.5V recessive voltage). Removed the electronic fan assembly and found obvious burn marks on the controller's internal PCB; the CAN transceiver chip (TJA1042 or compatible) had surface cracks. Analysis determined the fan motor stalled, causing internal overcurrent damage to the controller. After replacing the electronic fan assembly, communication returned to normal. Active testing confirmed all fan speed settings operated normally.
BYD DTC AI Analysis

AC controller software version defect causing false communication fault

✓
A dealership reported multiple 2019 Qin EVs intermittently logging DTC U012E87 after rain or car washing. Technicians found no faults in the wiring harness or fan assembly, and the fault cleared automatically. BYD factory technical support confirmed that early AC controller software (below V1.2) has an overly sensitive communication fault tolerance mechanism that falsely detects communication loss during 12V power system voltage fluctuations (such as when operating wipers or spraying washer fluid). After upgrading the AC controller to V1.5 (optimizing communication timeout logic) and monitoring for one month, the fault has not reoccurred.
BYD DTC AI Analysis

Chafed wiring harness caused intermittent CAN_L short to ground

✓
When driving on rough roads, the dashboard occasionally displayed fault warnings; operation remained normal on smooth surfaces. DTC U012E87 (history/intermittent) present. Close inspection revealed the electric cooling fan wiring harness had chafed through at the securing clip due to prolonged vibration, damaging the insulation and exposing the CAN_L wire's copper strands. This contacted the body metal, causing a short to ground. Cut out the damaged harness section and soldered in replacement wire of identical specification. Applied double-layer heat shrink tubing for insulation. Rerouted the harness and fitted anti-chafe sleeving, ensuring clearance from sharp edges. Road testing confirmed the repair.
BYD DTC AI Analysis

Loose ground bolt on the cooling fan caused communication failure.

✓
DTC U012E87 appeared after accident repairs. Inspection found that during collision repair, removal and refitting of the front wiring harness left the electric fan earth wire (normally a thick black cable) mounting bolt loose, resulting in 15Ω earth resistance. Although the fan could sometimes run at low speed (completing the circuit through casing contact), the poor ground caused the controller's internal communication circuit to malfunction, distorting CAN signals. The technician sanded the paint from the earth point, applied conductive paste, and tightened the bolt to standard torque (normally 8-10 Nm). Earth resistance dropped to 0.1Ω. The fault code cleared and communication returned to normal.
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: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.