Skoda U1123 is a Volkswagen Group databus fault indicating that a control module has received a CAN or LIN bus value from a sending module that is outside the expected plausible range — the data is received but is implausible or invalid.
What U1123 means
U1123 is a VAG-platform (Volkswagen/Skoda/Audi/SEAT) generic network fault code that appears in the fault log of the receiving module when it gets a data message from another module on the CAN or LIN bus, but the value in that message is outside its calibrated expected range. Unlike a full communication loss (which would generate a U0XXX no-communication code), U1123 means the bus connection is alive and frames are being exchanged, but the specific signal value received is flagged as implausible. Common scenarios: the instrument cluster receives engine speed from the ECU but the value arrives as 0 RPM while the engine is running; the Gateway module receives a CAN frame with a checksum error; or a sensor value arrives outside its physical limits. On Skoda Octavia, Superb, Kodiaq, and Karoq, U1123 appears most often in the Central Electronic Module (J519) or Gateway (J533) fault logs and points to a CAN bus signal integrity issue or a failing sending module.
Symptoms
- U1123 stored in one or more control modules (typically Gateway, Central Electronics, or Instrument Cluster)
- One or more vehicle functions may be erratic or display incorrect values depending on which signal is implausible
- Warning lamps may appear on the instrument cluster for the system associated with the implausible signal
- May occur intermittently, with symptoms disappearing and returning between drive cycles
- U1123 may appear alongside companion U0XXX communication loss codes if the bus fault is progressive
Common causes
- Failing or intermittent sensor sending data outside its valid range — for example a faulty ambient temperature sensor, steering angle sensor, or wheel speed sensor
- CAN bus wiring fault causing signal corruption — a damaged twisted-pair CAN wire introduces noise that corrupts the data value in the received frames
- Termination resistor fault or missing CAN bus termination causing signal reflections that distort message values
- Water ingress into a control module or connector causing intermittent CAN bus short circuits
- Software version mismatch between modules after an update, where one module sends a signal format the receiving module's older software does not recognise as valid
- Failing module (ECU, TCU, or sensor ECU) generating out-of-range output values as its internal circuitry degrades
Diagnostic approach
- Read U1123 with VCDS or ODIS and note which module stores the fault and which signal is implausible — Connect VCDS or ODIS and read all module faults with a full vehicle scan. U1123 entries in VCDS include additional detail identifying the sender (SPN — Suspect Parameter Number) and the specific implausible value condition. Note the storing module (e.g. J519, J533) and the sending module/signal identified in the fault detail — this directly points to the area to investigate.
- Check for companion U0XXX codes and CAN bus integrity — U1123 alongside U0001 (High Speed CAN bus fault) or U0100/U0101 (specific module communication loss) indicates a physical CAN bus problem — inspect wiring for damage. U1123 alone without U0XXX codes points more toward a sensor or module sending bad data rather than a bus wiring fault.
- Monitor the suspect signal in live data and check for out-of-range values — Use VCDS measuring blocks or live data to monitor the signal identified in the U1123 fault detail. For example, if U1123 is linked to the ambient temperature signal, watch the ambient temperature value in live data while driving — a value jumping to -40C or +80C randomly confirms a faulty ambient temperature sensor. Replace the identified sending component.
Make & model notes
Skoda: Octavia IV (2020-2025): U1123 on this model most often appears in the J519 (Central Electronics Module) fault log and is associated with the ambient temperature sensor or steering angle sensor signal going out of range. The ambient temperature sensor on the Octavia IV is located in the driver side mirror housing and can fail from road debris impact.
Skoda: Kodiaq (2017-2023): U1123 on the Kodiaq is frequently associated with the trailer detection or tow bar control unit sending an implausible signal when no trailer is connected. Check for a tow bar wiring fault or incorrect tow bar coding if U1123 appears on a Kodiaq with aftermarket tow bar installation.
FAQ
What does 'value received' mean in Skoda U1123?
It means the receiving module got a data message from another module over the CAN bus, but the number in that message was outside the range the receiving module considers physically possible. For example, if the expected engine coolant temperature range is -40C to +150C, a message arriving with a value of +200C would trigger U1123 as an implausible received value.
Is U1123 on Skoda fixed by a software update?
Sometimes. If U1123 is caused by a software version mismatch between modules (one module sending a data format the receiver's older software cannot validate), a software update to align versions can resolve it without hardware replacement. ODIS at a Skoda dealer can identify whether a software update is available for the affected modules.
Can VCDS alone diagnose and fix U1123 on Skoda?
VCDS is the recommended tool for initial diagnosis — it reads U1123 with the SPN detail needed to identify the implausible signal, and it provides live data to monitor the suspect value. However, the underlying repair (sensor replacement, wiring repair, or module coding) must be performed after diagnosis. VCDS cannot resolve a hardware fault on its own, but it significantly narrows down where to look.