Fault code U1003 – a step-by-step guide to diagnosing and fixing it

Error code U1003 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system detects a communication error in the CAN bus network (Controller Area Network), which affects data transmission between different control units in the vehicle. This DTC mainly affects the network communication between engine, transmission, ABS and other electronic control modules. This error code can have different specific meanings for different manufacturers, but basically refers to CAN bus faults.

What causes can this error code have?

• Defective CAN bus wiring
• Damaged or corroded plug connections
• Short circuit in the CAN bus cable
• Interruption of the CAN bus connection
• Defective control unit in the network
• Faulty terminating resistors
• Voltage problems in the on-board power supply
• Electromagnetic interference
• Water damage to electronic components

What symptoms occur in parallel with error code U1003?

• Communication problems: Various vehicle systems do not function properly or fail completely
• Warning lights: Multiple indicator lights illuminate simultaneously on the dashboard
• Starting problems: The vehicle may not start or the engine may run rough
• Transmission problems: Automatic transmission does not shift correctly or remains in emergency program
• ABS faults: Anti-lock braking system and ESP no longer work
• Instrument failure: speedometer, rev counter or other displays work intermittently

How to fix the problem of error code U1003?

• The repair requires a systematic diagnosis of the CAN bus network using professional diagnostic equipment.
• Perform complete CAN bus diagnostics: Use a professional OBD scanner to check all control units for communication errors and identify defective modules
• Check wiring and plug connections: Check all CAN bus cables for damage, corrosion or loose connections and repair or replace if necessary
• Measure terminating resistors: Check the 120 ohm terminating resistors at both ends of the CAN bus with a multimeter and replace defective resistors
• Test control units individually: Test each control unit connected to the CAN bus in isolation to identify and replace the module causing the fault
• Check the vehicle electrical system voltage: Check battery and alternator for correct voltage supply, as voltage fluctuations can cause CAN bus errors
• Eliminate sources of electromagnetic interference: Identify retrofitted electronic devices or defective components that could cause interference in the CAN bus