AU
U012604

DTC U012604 indicates a CAN bus communication interruption or data frame reception timeout between the Steering Angle Sensor (SAS) and the ABS/ESP control unit — Atto 3

Braking System

DTC U012604 indicates a CAN bus communication interruption or data frame reception timeout between the Steering Angle Sensor (SAS) and the ABS/ESP control unit.

From a network communication perspective, the ESP module logs this fault if it fails to detect a valid CAN message (whose ID typically contains steering angle, angular velocity, and fault status signals) from the SAS within the preset monitoring period (typically 100-200ms).

The SAS is the core sensor of the vehicle stability control (ESC/ESP) system, providing driver steering intent information.

Communication failure prevents the ESP from calculating the yaw moment correction, triggering system downgrade protection.

ESC/ABS functions may completely deactivate or enter limp mode.

This failure simultaneously affects ADAS functions relying on the steering angle signal, such as the Electronic Parking Brake (EPB), Automatic Emergency Braking (AEB), and lane keeping.

This powertrain CAN (PT-CAN) or chassis CAN (Ch-CAN) network communication fault is typically sporadic and intermittent.

Loose wiring connections, electromagnetic interference, or intermittent module lockups commonly cause this issue.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Abnormal SAS module power supply or ground: Includes a blown fuse, power supply circuit voltage out of range (less than 9V or greater than 16V), or oxidized ground terminals causing excessive contact resistance (greater than 1Ω), resulting in intermittent module resets or unstable CAN transceiver operation.
  • 2CAN bus physical layer fault: CAN-H and CAN-L line open or short circuit (shorted together, or shorted to power/ground), loose connection or backed-out pin at a wiring harness junction (such as the instrument panel shorting connector), terminating resistor drift (normal: 60Ω; abnormal: below 40Ω or above 80Ω)
  • 3Steering wheel angle sensor module fault: internal MCU crash, damaged CAN transceiver chip, sensor element failure (photoelectric encoder or magnetoresistive element fault), or software version defect causing periodic communication interruption.
  • 4Network electromagnetic interference: Aftermarket high-power devices such as non-genuine dash cams, inverters, and modified audio systems interfere with the CAN signal through the power supply or radiated emissions, or plug-in OBD devices occupy CAN bus bandwidth.
  • 5ESP/ABS control unit receiver fault: An internal CAN controller fault in the ESP module, a software parsing error, or an incorrect gateway module routing strategy causes SAS data packet loss.
What To Do
  • 1
    Fault confirmation and freeze frame analysis: Use VDS2000/Launch X431 to read all fault codes. Confirm whether U012604 is a Current or History code. Record freeze frame data, including vehicle speed, steering angle, and supply voltage. Clear the fault codes and road test the vehicle to reproduce the fault.
  • 2
    SAS module power and ground measurement: Disconnect the battery negative terminal. Unplug the SAS module connector (located below the steering column). Measure the voltage to ground at PIN1 (constant B+) and PIN2 (ignition switch power IGN) (should be 12V±0.5V). Measure the resistance to ground at PIN3 (ground) (should be less than 0.5Ω). Check the connector for water ingress or oxidation.
  • 3
    CAN bus static test: Switch off the ignition and wait 3 minutes for the bus to enter sleep mode. Measure the resistance between diagnostic connector OBD PIN6 (CAN-H) and PIN14 (CAN-L). The resistance should be approximately 60Ω (two 120Ω terminating resistors in parallel). If the resistance is abnormal, measure circuit continuity in sections between the ESP and SAS.
  • 4
    CAN bus dynamic waveform check: Measure the CAN waveform at the SAS connector using an oscilloscope. A normal waveform shows a symmetrical 2.5V reference voltage, with CAN-H pulled up to 3.5V and CAN-L pulled down to 1.5V. If flat-topped waveforms, noise, or insufficient amplitude occur, check the wiring harness shielding for damage and locate interference sources.
  • 5
    Wiring harness junction inspection: Remove the lower dashboard trim panel and inspect the CAN line shorting connector above the accelerator pedal and near the steering column (such as the crimp terminal in Case 2). Verify all wires seat fully into the metal terminals without backed-out pins, cold solder joints, or exposed copper strands. Re-crimp or solder if necessary.
  • 6
    SAS module replacement and calibration: After replacing the SAS module, use the diagnostic tool to perform the 'Steering Angle Sensor Calibration' procedure (this usually requires centering and securing the steering wheel, or triggering automatic calibration via the diagnostic tool). Clear the fault codes and perform a road test to confirm the ESP and ABS warning lights are off and the steering angle value in the data stream responds normally.
Real-World Cases
BYD DTC AI Analysis

BYD S7 intermittent ESP warning light and multi-module communication fault

✓
Customer reported the ESP warning light illuminates intermittently with no set pattern, sometimes suddenly lighting up while driving without affecting vehicle behaviour. Connected a scan tool and retrieved U010004 (EMS timeout, historical fault) and U012604 (SAS CAN communication timeout, historical fault) from the ESP module, plus U1701 (TCU-ECM communication failure) from the TCU. The engine system showed no current DTCs. This is an intermittent historical fault that is difficult to reproduce after clearing. Check the steering angle sensor power and ground circuits for stability, check the SAS module for intermittent faults, and verify CAN network wiring integrity, particularly for fatigue fractures in the harness near the steering column caused by steering movement.
Original source ↗
BYD DTC AI Analysis

Loose connection at the CAN bus splice point caused complete network failure on the all-new generation BYD Tang petrol.

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Vehicle developed intermittent no-start faults (two occurrences, six months apart). During failure, the electric cooling fan ran continuously, the instrument cluster displayed "Check Vehicle Network System", the P gear indicator flashed continuously, and warnings appeared for the engine, ESP and brake systems. VDS scan revealed multiple module communication faults: ESP logged U012604 (SAS CAN communication timeout), U100304 (EPB communication timeout) and U014604 (gateway communication timeout); the ECM logged U0164 (AC controller communication timeout); EPB logged U010187 (TCU communication timeout), U010087 (ECM communication fault) and U012287 (ESP communication fault). CAN line voltage and resistance measured normal, ruling out water intrusion from window film installation and dash cam interference. The fault reoccurred when tugging the instrument panel wiring harness, leading to the CAN splice connector above the accelerator pedal. Stripping back the harness revealed one CAN wire at the CAN-H splice connector was not crimped into its terminal, causing poor contact. Re-stripped the CAN-H splice connector, properly crimped the missed wire and secured all connections. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM: Intermittent Full Dashboard Illumination and Power Steering Failure

✓
While driving, all warning lights on the instrument cluster intermittently light up, the speedometer drops out, and steering loses power assist (goes heavy). Restarting does not clear the fault, but functions may recover after sitting parked. VDS scans show ECM, TCU, instrument cluster, drive controller, BUS, EPS, ESP, and other modules simultaneously reporting communication errors. This is a CAN network paralysis fault, the same category as the network communication fault in Case 2. U012604 (ESP subsystem communication loss) typically accompanies this fault. Repair: Inspect the CAN bus backbone lines, gateway controller, and module power and ground connections. Check harness junction points inside the instrument panel and wiring around the steering column for wear. Replace damaged harness sections and retest the network topology.
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.