What does it mean?
OBD II fault code P0863 is a generic code that is defined as “Transmission control module (TCM) communication”, and is set when communication between the TCM (Transmission Control Module) and the PCM (Powertrain Control Module) or other control modules is lost. Modern vehicles can have up to two dozen or more control modules, all of which are interconnected via the CAN (Controller Area Network) bus system.
In simple terms, the CAN bus system consists of thousands of circuits and connections, and its function is to carry information to and from all control modules that are engaged in a particular function, such as the control/management of transmission shift patterns.
In practice, the TCM gathers information from a host of sensors and other control modules, such as the engine load/speed sensor(s), throttle position sensor, steering angle sensor and others, which information it communicates to the PCM via the CAN bus system.
The PCM then uses this input data to calculate shift points before making suitable adjustment to the engine/fuel management systems in order to both modulate the engine power output to make gearshifts smoother, and to protect transmission components. In terms of operation, all control modules have at least one other, closely related module to which it regularly reports its state of health via the CAN bus system.
Thus, if a control module, in this case, the Transmission Control Module fails to report its state of health, the “companion” control module will set a relevant trouble code for the non-reporting control module. Therefore, when input data from the TCM and/or other control modules is lost, the PCM is unable to control gearshift patterns effectively, and it will set code P0863 and illuminate a warning light as a result.
In some cases, the PCM may initiate a failsafe or limp mode to limit engine power and to protect the transmission.