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 8

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 8
  • 1Abnormal SAS module power supply or ground: Includes a blown fuse, excessive voltage drop in the power supply circuit (less than 9V or greater than 16V), or oxidized ground terminals causing excessive contact resistance (greater than 1Ω), which causes 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, to power, or to ground), poor connection or backed-out pin at a wiring harness junction (such as the instrument panel joint connector), or terminating resistor drift (normal: 60 Ω; abnormal: below 40 Ω or above 80 Ω).
  • 3Steering wheel angle sensor module fault: internal MCU crash, CAN transceiver chip damage, 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 external OBD devices occupy CAN bus bandwidth.
  • 5ESP/ABS control unit receiver fault: ESP module internal CAN controller failure, software parsing error, or incorrect Gateway module routing strategy causing 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 power 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 power B+) and PIN2 (ignition 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 check: Turn off the ignition and wait 3 minutes for the bus to enter sleep mode. Measure the resistance between OBD PIN 6 (CAN-H) and PIN 14 (CAN-L) at the diagnostic connector. The resistance should be approximately 60 Ω (two 120 Ω 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 displays a symmetrical 2.5V reference voltage, with CAN-H pulled up to 3.5V and CAN-L pulled down to 1.5V. If the waveform shows flat tops, noise, or insufficient amplitude, inspect the wiring harness shielding for damage and check for interference sources.
  • 5
    Inspect wiring harness junction points: 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 fully seat into the metal terminals, showing no 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.

✓
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 8 Fault Codes
B1108For 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.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.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
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.