AU
U025487

U025487 (U0254-87, where 87 is a BYD custom sub-code meaning "signal/information missing or incorrect") indicates interrupted CAN communication between the air conditioning control module (ACU) and the PTC (Positive Temperature Coefficient) heater controller — Atto 3

Thermal Management System

U025487 (U0254-87, where 87 is a BYD custom sub-code meaning "signal/information missing or incorrect") indicates interrupted CAN communication between the air conditioning control module (ACU) and the PTC (Positive Temperature Coefficient) heater controller.

The PTC is the core high-voltage component of the new energy vehicle heating system (typically operating at 200-750V, 3-6kW).

It converts electrical energy into heat, replacing conventional engine waste heat.

This fault means the ACU cannot send power regulation commands (PWM or CAN signals) to the PTC or receive PTC feedback regarding high-voltage interlock status, IGBT temperature, operating current, and fault codes.

This causes complete failure of the air conditioning heating function (no warm air).

In low-temperature environments, this may affect front windshield defrosting and cabin heating, but typically does not affect vehicle drive functions.

The communication interruption occurs because the ACU fails to receive a valid CAN frame from the PTC node (Node ID typically in the 0x180-0x1FF range) for 3-5 consecutive message cycles (typically 200-500ms), triggering a timeout.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Abnormal PTC controller low-voltage power supply: Includes a blown 12V constant power (B+) fuse (usually F2/15A in the front compartment fuse box), a faulty ignition power (IGN) relay, or a loose or oxidized ground point (G301/G302 on the front compartment side member), preventing the controller from initializing the CAN transceiver.
  • 2CAN network physical layer fault: Short circuit, open circuit, or poor connection in the CAN-H (orange/black) and CAN-L (orange/brown) wiring harness of the air conditioning sub-network (Comfort CAN or HVAC CAN), or 120Ω terminating resistor drift or detachment, causing signal reflection or bus failure.
  • 3PTC controller internal fault: Damaged internal CAN transceiver chip (e.g., TJA1041/1051), failed 3.3V/5V power management IC, or crashed main control MCU unable to respond to bus requests.
  • 4Air conditioning control module (ACU) fault: Damaged ACU internal CAN communication interface circuit, or outdated software causing a communication protocol mismatch with the PTC controller (common after upgrades on 2018-2019 Qin Pro DM models).
  • 512V battery voltage too low: When voltage drops below 10.5V, the PTC controller may fail to initiate CAN communication, especially during vehicle startup, or due to battery aging or excessive parasitic draw.
What To Do
  • 1
    Fault Confirmation and Freeze Frame Analysis: Use the VDS2000/3000 or a dedicated BYD diagnostic tool to read the fault code and confirm if U025487 is a current fault (Active). Record freeze frame data, including ambient temperature, PTC coolant outlet temperature, and high-voltage status. Determine if the fault occurs only during high-voltage power-up or when switching on the heater.
  • 2
    Basic power supply and ground check: Disconnect the PTC controller low-voltage connector (usually 8-12 pins, located near the heater core). Measure the voltage to ground at pin 1 (B+ constant power) and pin 2 (IGN); the voltage must be 12.0-14.5V. Measure the resistance to ground at pins 4/5 (GND); the resistance must be <1Ω. Check the PTC fuse in the front compartment fuse box and inspect the ground points on the left and right front longitudinal beams for oxidation.
  • 3
    CAN line physical layer check: At the PTC controller connector, measure CAN-H to ground voltage (2.6-3.0V), CAN-L to ground voltage (2.0-2.4V), and resistance between CAN-H and CAN-L (parallel network should be approximately 60Ω; disconnected module measurement should be 120Ω). Observe the waveform using an oscilloscope. A normal waveform is a square wave with a 2V amplitude. Flat tops, distortion, or common-mode voltage shifts indicate a short circuit.
  • 4
    Harness continuity and insulation test: Disconnect the motor compartment harness from the PTC controller. Measure the CAN line continuity resistance from the ACU to the PTC (<1 Ω). Perform insulation tests to ground and power (>1 MΩ). Inspect the harness near the front motor compartment bulkhead and the PTC unit connector for high-temperature aging or coolant leak corrosion.
  • 5
    High-voltage interlock and controller substitution test: Verify the high-voltage manual service disconnect (MSD) is connected. Measure the PTC high-voltage input voltage (must be within the traction battery voltage range). If a high-voltage interlock fault accompanies the communication fault, check the interlock circuit continuity at the PTC high-voltage connector. If normal, swap the PTC controller to test. If the fault transfers, replace the PTC assembly. If communication still fails, check or replace the ACU and update the software.
Real-World Cases
BYD DTC AI Analysis

BYD Han EV Steering Column Control Module Communication Loss (U0212) - CAN Intermittent Connection Diagnostic Reference

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Symptoms: 2020 Han EV instrument cluster intermittently displayed "Check vehicle network", accompanied by intermittent power steering failure and abnormal steering angle sensor data stream. Diagnosis: VDS showed Steering Column Control Module (SCCM) communication timeout—code U0212 (same communication category as U025487). Static voltage at the OBD port measured 2.6V on CAN-H and 2.4V on CAN-L, but shaking the wiring harness beneath the steering column caused 0.5V fluctuations. The 16-pin connector had slightly oxidized terminals and a loose retaining clip. Resolution: Cleaned the terminals with electronic contact cleaner and restored the connector clip tension. Fault eliminated. This case demonstrates the 'poor connection/oxidation' diagnostic approach for CAN communication faults, closely matching PTC communication troubleshooting logic.
Original source ↗
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BYD S6 transmission mechatronic unit communication fault (loss of TCU communication) — poor ground

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Symptoms: 2013 S6. Gearbox warning light illuminated after 30 minutes of high-speed driving, followed by jerking during deceleration and limp mode. Diagnosis: DTCs indicated the gearbox mechatronic unit (TCU) was not responding. Voltage testing revealed a 0.8V drop between the battery negative terminal and body ground strap, confirming poor ground caused unstable communication voltage. Further inspection showed the gearbox wiring harness connector seal (at the firewall) had deteriorated, allowing coolant ingress and terminal corrosion. Solution: Replaced the gearbox wiring harness connector, cleaned and tightened the body ground point, and replaced the mechatronic unit. This case shows that poor power supply grounding can interrupt control module communication. When troubleshooting U025487, check the PTC controller ground point.
Original source ↗
BYD DTC AI Analysis

BYD Qin DM Lost Communication Between BMS and High-Voltage Battery Management Module (U0111) - Wiring Harness Damage

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Symptoms: 2015 Qin DM instrument cluster displayed 'Check Powertrain', EV mode failed, and the vehicle would not charge. Diagnosis: Read BMS DTC U0111 (Lost Communication With High Voltage Battery Manager). Measured CAN lines between BMS and BIC; 12V constant power at the BIC was normal, but the wake-up signal intermittently disappeared. Traced the circuit to the right front fender liner and found the harness pinched during prior accident repair, causing an intermittent open in the wake-up line. Resolution: Repaired the damaged harness (re-soldered and sealed with heat-shrink tubing). Rechecked CAN termination resistance: 120Ω, normal. This case shows an intermittent communication fault caused by physical harness damage. For U025487, check the wiring harness between the PTC controller and ACU.
Original source ↗
BYD DTC AI Analysis

BYD Qin EV PTC Communication Fault (U025487) - Connector Coolant Corrosion

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Symptoms: 2019 Qin EV instrument cluster displayed 'Air Conditioning System Fault' after heater activation in winter. Scan tool read active DTC U025487. Diagnosis: 12V supply to PTC controller measured normal, but CAN-H voltage read only 1.2V. Inspection of PTC controller connector (located above heater core) found terminals oxidised green inside the plug from long-term coolant vapour erosion. Contact resistance measured 5Ω. Sealing ring missing. Solution: Cleaned terminals with electronic contact cleaner, applied conductive grease, replaced waterproof sealing ring and fitted heat shield. Cleared fault codes. Heater function restored; CAN voltage returned to normal.
BYD DTC AI Analysis

BYD Qin Pro DM PTC intermittent communication loss — Loose ground point

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Symptoms: 2018 BYD Qin Pro DM cabin heater suddenly cuts out on rough roads. Intermittent DTC U025487; normal on flat roads. Diagnosis: Freeze frame shows fault occurred crossing speed bumps. PTC controller ground wire measured 0.5Ω static, but jumped above 15Ω when shaking the harness. Front left longitudinal beam ground point (G301) bolt loose (<3Nm); ground wire internal copper strands 90% fractured, only a few remaining connected. Solution: Replaced ground wire harness, sanded the ground point, and tightened the bolt to 8-10Nm. No recurrence.
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.