What does it mean?
OBD II fault code U0402 is a generic code that is defined as “Invalid Data Received from TCM”, and is set when the PCM (Powertrain Control Module) detects a signal from the TCM (Transmission Control Module) that is inaccurate, implausible, invalid, or falls outside of the expected range. All automatic transmissions use some form of computerized control in conjunction with the PCM and other control modules both to make gearshifts smoother, and to improve overall fuel economy.
While the actual gearshifts on an automatic transmission are performed by electrically operated solenoids that shunt pressurized transmission fluid between fluid passages in the valve body, it is the transmission control module that directs the operation of the shift solenoids. In practice, it is the PCM that calculates shift points and patterns based on input data from a variety of engine and drivability sensors.
When a shift point is reached, the PCM communicates the information to the TCM, which then completes the commanded gearshift by commanding specific solenoid valves to redirect pressurized fluid out of, and into another fluid passage. By pressurizing different passages at different times, the transmission is able to deselect one gear ratio, before selecting another by reconfiguring a set of clutches and planetary gears.
Note that the TCM communicates its actions back to the PCM via a dedicated feedback circuit. However, the TCM is connected to the PCM and other control modules such as the ABS control module through the CAN (Controller Area Network) bus serial communications system via hundreds of circuits and sometimes thousands of connections.
The reason for this arrangement is that in cases where the PCM determines that a dangerous condition exists (based on input data from a host of sensors) and that the vehicle is approaching the limits of controllability, it needs to be able to prevent the transmission from shifting up or down, which could aggravate the situation.
Moreover, both the traction-, and stability control systems depend on the transmission not to execute gearshifts at the wrong time, such as when the traction control system is attempting to correct wheel spin on a slippery surface. In these cases, the PCM will calculate the appropriate gear ratio, and will prevent the transmission from shifting out of that ratio until it determines that wheel spin is no longer occurring, and that the vehicle is accelerating normally and safely.
From the above it should be obvious that for the PCM, TCM and other control modules to work together effectively, the PCM needs to receive accurate and valid input data from the transmission control module at all times. Thus, when the PCM detects input data from the transmission control module that is invalid given the current operating conditions, it will set code U0402, and illuminate a warning light.
Note that on most applications, the PCM will also initiate a failsafe or limp mode both as a safety measure, and to protect the transmission.