What does U0402 mean?

U0402 is stored when a module receives data from the transmission control module (TCM) that it cannot accept — values that are out of range, contradictory, or implausible. The important distinction is that communication itself is working. The TCM is on the bus and talking, so this is not a lost-communication fault and it is not proof that the transmission is failing. Three explanations come first. One is a bad input feeding the TCM: speed sensors, range sensors, pressure switches, and temperature sensors all supply the numbers the TCM broadcasts, and a failing sensor or a shorted signal wire makes the module publish values the rest of the network rejects. That is why reading every code in every module matters — an accompanying transmission P-code is very often the real fault, and U0402 is the downstream symptom. Two is software and configuration: mismatched calibrations, an interrupted programming session, a used module that was never configured to this vehicle, or an outstanding calibration update will all produce data the network refuses. Those cases are fixed with programming, not parts. Three is the electrical foundation shared by every U-code: weak battery voltage, a corroded ground, a damaged connector, or bus wiring that corrupts messages in transit. Transmission harness connectors live in a hot, wet, vibrating place, so water intrusion and spread pins are common and cheap to find if you look. Work in order: test the battery and charging system, read all codes everywhere, verify the module's software and programming status, then inspect grounds, connectors, and the wiring of any sensor a related code names. Expect limp mode, harsh or delayed shifts, and a check-engine light while the code is active, and keep driving to a minimum — limp mode exists to protect the transmission, not to let you carry on normally. Replacing the TCM belongs at the end: it requires programming and sometimes an adaptive relearn, and it will not fix a sensor or calibration problem.