Fault code U1123 – a step-by-step guide to diagnosing and fixing it

Error code U1123 is displayed and the check engine light is on - what does this mean?

The DTC U1123 is recognized by the OBD system and indicates a communication problem in the CAN bus network, especially between different control units. This error code relates to data transmission between electronic components and may have slightly different definitions for different manufacturers, but usually refers to network communication faults.

What causes can this error code have?

• Defective CAN bus cabling
• Damaged plug connections in the network
• Failed control unit (ECU)
• Corroded contacts on the connections
• Short circuit in the data line
• Defective CAN bus resistor (terminator)
• Voltage problems in the vehicle electrical system
• Faulty gateway modules
• Damaged shielding of the data lines

What symptoms occur in parallel with error code U1123?

• Communication failure: Different systems cannot communicate with each other
• Malfunctions: Individual vehicle functions are not working properly
• Warning lights: Multiple indicator lights in the dashboard illuminate
• System failures: Complete failures of electronic systems
• Lack of data transmission: Diagnostic devices cannot connect to certain control units
• Intermittent problems: Sporadic failures of various electronic components

How to fix the problem of error code U1123?

• The repair requires systematic diagnosis of the network communication in the vehicle.
• Check and repair the CAN bus wiring: Check all data lines for damage, breaks or short circuits and replace defective cables
• Clean and secure plug connections: Remove, clean and firmly reinsert all relevant connectors in the CAN bus network
• Carry out ECU diagnostics: Test each connected ECU individually and replace defective modules
• Measure and replace bus resistance: Check the terminating resistors in the CAN bus system and install new terminators if necessary
• Check the power supply: Check and correct the vehicle electrical system voltage and ground connections of all components involved
• Replace gateway module: If the gateway is defective, replace the central communication module with a new one