The Transmission Control Module (TCM) provides transmission and component performance information to the ECM (Engine Control Module). The data is sent over a communication network known as a CAN (Local Area Network) bus. This network uses two circuits to carry out the data communication between the ECM and the TCM. It is important to mention that, a CAN bus failure will not cause the direct setting of code U0402, rather, if the network fails, it will set another DTC that relates to the problem.
When the U0402 OBDII diagnostic code is stored, it means that the ECM has received data from the TCM, which is not valid.