U0402: OBD-II code U0402 flags that another module received invalid or implausible data from the TCM — the network works but transmission messages are wrong.

Symptoms

• Check engine light is on; harsh, delayed, or unpredictable shifts because other modules can't trust the TCM's data.
• Stability control may behave incorrectly during shifts because torque coordination between TCM and PCM breaks down.
• TCM companion codes typically accompany U0402 — the TCM has an internal issue producing the bad data.

Common causes

• A failing TCM broadcasting bad values on the bus — usually paired with internal TCM codes.
• A software mismatch between the TCM and other modules after a partial reflash or replacement.
• Contaminated transmission fluid producing bad sensor data that the TCM then relays to the network.

What to do?

Scan the TCM for its own codes first — U0402 is usually a downstream symptom. Fix TCM internal faults and address transmission fluid issues (level and condition). Verify all modules are on matching software versions. Reprogram or replace the TCM if bad values continue after other repairs.