OBD-II code U0679 indicates that the powertrain control module (PCM) has lost communication with the transmission fluid pressure sensor or switch on the vehicle's communication network. This typically means the sensor or its circuit is not sending data as expected, which can affect transmission operation and shift quality.
The sensor itself may be defective, sending incorrect or no signals to the communication network. This is the most common cause of U0679.
Damaged, corroded, or loose wiring or connectors between the sensor and the control module can disrupt communication. Shorts or open circuits are common culprits.
A weak or corroded ground connection can cause intermittent or complete loss of communication between the sensor and the network.
Problems with the Controller Area Network (CAN) bus, such as damaged wiring, shorts, or high resistance, can prevent proper data transmission between modules.
While less common, the TCM may fail to process or relay the sensor's data correctly, leading to a communication loss.
PCM failure is rare for this code and should only be considered after all other causes have been thoroughly ruled out. Replacing the PCM is unlikely to resolve U0679 unless there is clear evidence of module malfunction.
Driving with this code may result in poor transmission performance, harsh shifting, or the vehicle entering limp mode. It is recommended to diagnose and repair the issue as soon as possible to avoid further damage.
No, the PCM is rarely the cause of U0679. This code typically stems from issues with the sensor, wiring, or communication network. Replacing the PCM should only be considered after all other potential causes have been ruled out.
Start by inspecting the sensor and its wiring for visible damage or corrosion. Use a multimeter to test the sensor's voltage, ground, and signal output. If the sensor tests within specifications, focus on the wiring and connectors.
Resetting the code may temporarily clear the check engine light, but it will not fix the underlying issue. The code will likely return if the root cause (e.g., faulty sensor, wiring, or communication problem) is not addressed.