Fault-code directory · Vehicle network diagnostics
Engine and transmission controllers exchange information to coordinate torque and gear changes. If one receives data it cannot use, the vehicle may limit performance or store several warnings. The controller reporting the disagreement is not necessarily the failed component.
Codes covered
U0401 · U0402
U0401 concerns data from the engine or powertrain controller; U0402 concerns data from the transmission controller. These differ from a complete loss of communication. A functioning network can carry data that is marked invalid because an input, operating condition or configuration is wrong.
Confirm the full description, suffix and reporting module for your vehicle. These are diagnostic starting points, not a parts replacement list.
Evidence that helps narrow the fault
- Read the engine and transmission reports together. Identify any primary sensor, supply or internal faults before treating the data warning as an independent failure.
- Record when the warning appears and whether there has been programming, a replacement controller or a drivetrain modification.
- Compare the relevant torque, speed and operating-state information using the correct diagnostic coverage. A generic code reader may not expose the parameters needed to explain the disagreement.
Choosing the next repair step
A wiring repair is appropriate when the evidence identifies a network or supply issue. Programming is appropriate only when the vehicle and software evidence justify it. Neither should be prescribed just because a U-code is present. A useful diagnostic report states which data is unusable and what underlying fault caused that state.
Is replacing the CAN wiring the first step for any U-code?
No. Some U-codes describe invalid information rather than a broken network. Establish the type of report and the health of the sender before deciding which circuit needs testing.
Talk through the fault with AXION
Send the make, model, year, full scan report and a short description of when the fault happens. Include recent repairs or programming attempts. If you do not have a scan, describe the symptoms in your own words.
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Related reading
Reference for generic code terminology: Snap-on diagnostic code reference. Use the manufacturer’s current service information for the actual vehicle and test procedure. This article provides general diagnostic guidance and is not a report of a particular customer repair.