AU
U040100

U040100 is a U-class network communication fault code indicating "Invalid data received from Engine Control Module (ECM)" — Atto 3

Braking System

U040100 is a U-class network communication fault code indicating "Invalid data received from Engine Control Module (ECM)".

In the BYD DM-i hybrid architecture, the IPB (Intelligent Integrated Brake System) receives real-time parameters from the ECM via the powertrain CAN bus—including engine speed, output torque, and operating status—to coordinate the distribution ratio between regenerative and mechanical braking.

If the IPB detects an ECM data frame checksum error, data outside the physically reasonable range, a communication protocol version mismatch, or a data update timeout, it logs "ECM data corrupted" and stores this fault code.

This fault can force the IPB into a degraded mode, resulting in regenerative braking failure, restricted ESC function, and abnormal brake pedal feel.

In extreme cases, it triggers a "Brake System Fault" warning and limits vehicle power output, severely compromising driving safety.

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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Poor contact in the ECM power supply/ground circuit: A loose fuse connection or oxidized/loose ground points G101/G102 causes unstable ECM operating voltage, resulting in the ECM transmitting corrupted data frames.
  • 2Powertrain CAN bus physical layer fault: Short or open circuit in the twisted pair between the ECM and gateway, terminal resistor drift (normal 60Ω), or connector pin oxidation (e.g., water ingress oxidizing the connector near the firewall).
  • 3ECM software fault: Flash data corruption due to interrupted flashing, software version mismatch with VCU/IPB (e.g., ECM is V1.02 and VCU is V2.1), or lost calibration data.
  • 4Gateway controller fault: A burnt capacitor on the internal gateway PCB or a processor failure corrupts or delays ECM data packets forwarded to the IPB.
  • 5ECM hardware fault: Damaged internal main processor (MCU), failed CAN transceiver chip, or corrupted EEPROM data causing continuous transmission of incorrect data frames.
What To Do
  • 1
    Safety pre-check and fault confirmation: Connect the VDS diagnostic tool to read all fault codes and freeze-frame data. Record the frequency and environmental conditions of U040100 (e.g., concurrent communication faults such as U0100). Check the instrument cluster warning light status. Confirm if the vehicle exhibits power limitation or abnormal braking.
  • 2
    Basic circuit check: Check ECM fuses (EF03/EF04, etc.), relays, and connector connections. Measure ECM supply voltage (constant B+ must be 12V±0.5V, IGN power normal). Measure resistance to ground at ground points G101/G102; resistance must be less than 1Ω. Check engine compartment wiring harness for wear or signs of water ingress.
  • 3
    CAN bus physical layer inspection: Disconnect the battery negative terminal. Measure the terminating resistance between OBD port pin 6 (CAN-H) and pin 14 (CAN-L) (standard value: approx. 60Ω; if abnormal, check the ECM or gateway terminating resistor). Check the CAN lines for short circuits to ground or power. Inspect the ECM connector pins for oxidation or blackening (focus on connectors near the firewall).
  • 4
    Communication network diagnosis: After restoring power, use VDS to perform a 'Network Test'. Observe the ECM communication status. Check the powertrain CAN bus waveform (CAN-H 2.5-3.5V, CAN-L 1.5-2.5V, sleep voltage 2.5V). Use an oscilloscope to check for signal interference or waveform distortion.
  • 5
    Software version check and update: Read the current ECM, VCU, and IPB software versions. Compare them against the latest BYD TIS version bulletin to check for version mismatches. If necessary, perform an ECM software update (via VDS2000 or OTA). Maintain stable voltage during the update (connect a regulated power supply). After completion, perform 'System Configuration and Calibration'.
  • 6
    Component isolation test: Disconnect the gateway and ECM connectors sequentially and measure the CAN resistance change to locate the faulty module. Perform a swap test using a known-good gateway or ECM. If the fault transfers after replacement, the corresponding module is faulty. For ECM hardware damage (e.g., accompanied by DTC P060C), perform offline programming or replace the ECM assembly, then complete immobilizer matching and parameter configuration.
Real-World Cases
BYD DTC AI Analysis

Oxidized wiring harness connector caused communication fault in Qin Pro DM

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Symptoms: While driving, the dashboard displayed "Check Power System", the check engine light stayed on, the vehicle automatically switched to petrol-only mode with restricted power, and EV mode was unavailable. The fault remained after turning the vehicle off and restarting. Diagnosis: VDS retrieved DTCs U040100 (Invalid Data Received from ECM) and U0100 (Lost Communication with ECM). Checked ECM power supply and ground – normal. Measured powertrain CAN bus voltage (CAN-H 2.6V, CAN-L 2.4V) – normal. Further inspection of the engine bay wiring harness found obvious oxidation and blackening on pin 15 of the ECM connector (located near the bulkhead), with contact resistance exceeding 5Ω, interrupting high-speed CAN signal transmission. Resolution: Cleaned the ECM connector pins with electrical contact cleaner, applied conductive paste, re-crimped the pins and applied waterproof sealant. Cleared the DTCs and road tested – communication restored, fault resolved.
Original source ↗
BYD DTC AI Analysis

Song Plus DM-i ECM Software Version Incompatibility

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Symptoms: After about 5000 km, the vehicle developed intermittent hard cold starts. Once started, engine speed fluctuated between 800–1500 rpm. The instrument panel fault light came on intermittently, especially after using cruise control. Diagnosis: Scanned DTCs and found only U040100. Checked the vehicle network topology and found data packet loss between the ECM and VCU (Vehicle Control Unit). Comparing configurations showed the ECM ran software version V1.02 while the VCU already ran V2.1. The protocol mismatch caused data frame checksum errors. Fix: Upgraded the ECM software to the latest version V2.1 (via BYD OTA or dealership VDS2000 flash). Ran the 'System Configuration and Calibration' procedure after the upgrade. The fault has not returned.
Original source ↗
BYD DTC AI Analysis

Tang DM gateway controller internal fault

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Symptoms: While driving, the vehicle suddenly displayed "Powertrain Severely Limited". Speed restricted to below 40 km/h; accelerator pedal unresponsive. Pulled over, shut down and restarted — fault persisted. Diagnosis: Scan tool showed DTC U040100 and communication faults U0101, U0121. Resistance between OBD pins 6 (CAN-H) and 14 (CAN-L) measured 45 Ω (normal: 60 Ω). Disconnected control modules one by one; resistance normalized only when the Gateway was unplugged. Disassembled the Gateway to find a burnt capacitor on the PCB, corrupting data packets sent to the ECM. Solution: Replaced Gateway controller (Part No.: LGX XX XXX). After replacement, performed Gateway Configuration Coding and Network Topology Learning using the scan tool. Cleared DTCs; road test normal.
Original source ↗
BYD DTC AI Analysis

Destroyer 05 ECM hardware fault (data corruption)

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Symptoms: After a routine software update at the dealership, the instrument cluster showed "Check Engine System." The scan tool read DTC U040100 with P060C (Internal Control Module Main Processor Performance). The vehicle could not shift into gear or drive. Diagnosis: The update logs showed communication interrupted during ECM flashing. The technician connected to the ECM with a diagnostic tool and established communication, but the data stream showed fixed values that would not change. The interrupted flash corrupted the ECM's internal Flash data. Physical layer communication remained normal, but the ECM transmitted data frames with incorrect formatting, causing the VCU to flag an "invalid data" fault. Resolution: Technicians performed offline programming on the ECM (using a dedicated programmer to rewrite the underlying data). After reinstalling the unit, they ran immobiliser matching and parameter configuration. Because some dealerships lack offline programming capability, the shop ultimately replaced the ECM assembly with a new unit to resolve the fault.
Original source ↗
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.