What it means
A control module has detected data from another module that it considers invalid or implausible. The code does not identify the source module; consult related codes and vehicle-specific diagnostic information.
How it works
Vehicle modules exchange messages such as operating status and sensor values over the communication network. U0400 is set when a receiving module rejects data as implausible, but the code itself does not identify which module sent it.
Common causes
Most likely first.
- Implausible data from a control module
- Software or configuration incompatibility
- Intermittent network data corruption
Symptoms
- A function using the rejected data may be disabled or operate incorrectly
- Related warning indicators may illuminate
- A diagnostic scan may show codes identifying the source module
How to diagnose it
- Scan every module and save freeze-frame data to find codes that identify the sender or the function affected when U0400 occurred.
- Compare live data from the suspected source module with related readings and the vehicle's operating state to see which value is implausible.
- Inspect the suspected module's connectors, grounds, and network wiring for loose pins, corrosion, or damage.
- Check for matching communication or voltage-supply faults in the source and receiving modules.
- If wiring and power checks pass, verify that the modules have compatible software and configuration, and investigate intermittent network faults.
Common fixes
- Repair a poor connector, ground, or network wire at the module identified by related codes.
- Address the underlying sensor or module fault causing implausible data.
- Correct incompatible module configuration or software where confirmed.
- Repair an intermittent network connection if communication faults accompany U0400.
Can I keep driving?
Whether it is safe to continue depends on the function rejecting the data; steering, braking, transmission, or other systems may be affected. If a safety warning appears or vehicle control changes, stop when safe and have the affected system diagnosed.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.