What it means
A control module has received data from the TCM that it considers invalid or implausible. Transmission operation or functions relying on TCM data may be affected; the specific effects depend on the vehicle.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
The transmission control module (TCM) sends gear, shift, and transmission status information to other vehicle modules. U0402 is set when a receiving module considers data from the TCM implausible or incompatible with the expected software or configuration.
Common causes
Most likely first.
- Implausible or incorrect TCM data
- Software or configuration incompatibility
- Intermittent network data corruption
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules and save freeze-frame data to identify which module logged U0402 and whether the TCM has related transmission or communication codes.
- Check live TCM data for gear selection, vehicle speed, and transmission status values that appear inconsistent with actual operation.
- Inspect the TCM connectors, grounds, and network wiring for corrosion, looseness, or damage.
- Check TCM power supply and network communication, and note whether other modules report lost TCM messages.
- If electrical checks pass, verify TCM software and configuration compatibility and diagnose any TCM faults indicated by related codes.
Common fixes
- Repair a loose, corroded, or damaged TCM connector, ground, or network wire.
- Correct the TCM configuration or update its software when a compatibility issue is confirmed.
- Repair the underlying TCM fault if its data remains implausible after wiring checks.
- Replace an incorrectly specified TCM and program the correct unit for the vehicle.
Can I keep driving?
Transmission shifts may become harsh, limited, or unpredictable, and the vehicle may enter a reduced-operation mode. Avoid towing, hard acceleration, and unnecessary driving; stop if the transmission will not select or hold a gear safely.
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.