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 — Seal U

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.

The U012604 fault code on the Seal U indicates a communication timeout between the Steering Angle Sensor and the ABS/ESP control unit. Severity is medium because ESC/ABS and some ADAS functions may be reduced or disabled. Common causes include poor sensor power or ground, or a CAN bus wiring/connector fault. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Abnormal SAS module power supply or ground: Includes a blown fuse, power supply voltage out of range (less than 9 V or greater than 16 V), or ground terminal oxidation resulting in excessive contact resistance (greater than 1 Ω), which causes intermittent module resets or unstable CAN transceiver operation.
  • 2CAN bus physical layer fault: CAN-H or CAN-L line open circuit, short circuit (shorted together or to power/ground), poor connection or backed-out pin at a wiring harness junction (such as the instrument panel shorting connector), or terminating resistor drift (normal 60 Ω, abnormal range below 40 Ω or above 80 Ω)
  • 3Steering angle sensor module internal fault: internal MCU crash, CAN transceiver chip damage, sensor element failure (photoelectric encoder or magnetoresistive element fault), software defect causing periodic communication interruption.
  • 4Network electromagnetic interference: Aftermarket dash cams, inverters, modified audio systems, and other high-power devices interfere with the CAN signal via the power supply or radiated emissions, or external OBD devices occupy CAN bus bandwidth.
  • 5ESP/ABS control unit receiver fault: Internal ESP module CAN controller fault, software parsing error, or gateway module routing strategy error 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 such as 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 negative battery terminal. Disconnect the SAS module connector (located below the steering column). Measure the voltage to ground at PIN 1 (constant power B+) and PIN 2 (ignition switch power IGN) (should be 12V ± 0.5V). Measure the resistance to ground at PIN 3 (ground) (should be less than 0.5Ω). Inspect the connector for water ingress or oxidation.
  • 3
    CAN bus static check: Turn off the ignition switch 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). Resistance should be approximately 60Ω (two 120Ω terminating resistors in parallel). If the resistance is abnormal, measure the wiring continuity between the ESP and SAS in sections.
  • 4
    CAN bus dynamic waveform check: Use an oscilloscope to measure the CAN waveform at the SAS connector. 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 flat-topped waveforms, noise, or insufficient amplitude appear, check the wiring harness shielding for damage and inspect for interference sources.
  • 5
    Inspect wiring harness junction points: Remove the lower dashboard trim panel and check 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, with no backed-out pins, cold solder joints, or exposed copper strands. Re-crimp or solder as necessary.
  • 6
    SAS module replacement and calibration: After replacing the SAS module, use the diagnostic tool to execute the 'Steering Angle Sensor Calibration' procedure (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 turn 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

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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

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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 Seal U Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U overview →
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.