|Code||Fault Location||Probable Cause|
|U0402|| Invalid data received - transmission control module (TCM) |
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|Wiring, Defective TCM, Defective transmission, PCM|
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What Does Code U0402 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.
Where is the U0402 sensor located?
The image above shows the location (arrowed) of the transmission control module on a Toyota Camry. Note that while the TCM is often located directly on or near the transmission, on many applications it can also be located in the engine compartment, behind the splash cover inside a front wheel well, under the dashboard, or even in a centre console.
If the module is not located on the transmission casing itself, refer to the manual for the affected application to locate and identify the transmission control module correctly, to avoid confusing the TCM with another module.
What are the common causes of code U0402?
Note that although code U0402 rarely indicates a faulty transmission, the possibility that the transmission is indeed defective should not be discounted altogether, and especially in cases where additional, transmission related codes are also present.
Nonetheless, some common causes of code U0402 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the TCM and associated harnesses. Note that if defects are found in the transmission’s internal harness, the entire internal harness must be replaced to avoid possible issues with short circuits or abnormal resistances later on
- Defective TCM
- Defective transmission
- Failed or failing PCM. Note that this is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced