What does it mean?
Audi, Nissan, and Volkswagen manufacturers define U1123 as SCP (J1850) Invalid or missing data for Odometer. The code is triggered due to a lack of communication or disconnection between controls and components that could affect the odometer. The most possible causes of this code include a malfunction in the VNT (Variable Nozzle Turbine) of the control unit and short circuits in communication wiring between these control units and the ECU.
U1123 is related to the Serial Corporate Protocol (SCP), also called the J1850. The J1850 is a standard protocol adopted by the US for Class B networks. It is defined as a system that transfers data between nodes to reduce redundant elements. It is one of the numerous communications protocols used by different auto manufacturers. The adoption of the SCP (J1850) makes communication between the control modules and other vehicle components possible by using BUS communication.
The code also refers to the odometer which measures the distance that is displayed on the dashboard and cannot be reset. The Odometer is controlled and regulated by modules like the ECM. Once there is a communication fault to the odometer, U1123 will be triggered. Note: I recommend inspecting the harness and connection for corrosion in diagnosing this code. Also, check for damaged pins and wiring. Verify the voltage to the ECU and flash its software and update it if possible.
Symptoms
- Odometer or trip display showing dashes, zeros or nothing at all
- Speedometer erratic while the odometer does not count up
- Mileage shown in the cluster and in other modules no longer agree when read with a scan tool
- Service interval reminders appearing at the wrong time or not at all
- Warning lamps in the cluster illuminated together with communication codes
- Intermittent blank display, especially in cold weather or over bumps
Likely causes
- Corroded or loose connector behind the instrument cluster, or pushed-back terminals
- Blown fuse, poor supply voltage or a bad ground at the cluster
- Open or shorted bus wiring between the cluster, the gateway and the engine control unit
- Cluster or control unit replaced without being coded and adapted to the vehicle, so the stored mileage does not match
- Water damage in the dash harness after a windscreen or heater leak
- Software mismatch between modules that store mileage after a programming session
- Internal fault in the instrument cluster
How to fix it
- Scan all modules and check which ones store a mileage value, then compare those values
- Inspect and reseat the cluster connectors, looking for green corrosion, bent pins and water marks
- Measure supply voltage and ground at the cluster with the circuit loaded, and check the relevant fuses
- Test the bus wiring between the cluster, the gateway and the engine control unit for continuity and shorts
- Wiggle test the dash harness while watching whether the display drops out
- Check for module software updates and confirm the coding of any recently replaced unit
- Repair or replace the cluster last, and always complete the correct coding and adaptation procedure afterwards