What does it mean?
OBD II fault code U0300 is a generic trouble code that is defined as U0300 – “Control Module – Internal Software Incompatibility” and is set when the PCM (Power train Control Module) detects a communication failure between two or more microprocessors located within the larger programming architecture of the PCM- hence the phrase “Internal Software”.
We need not delve into the complexities of electronic control unit programming architectures here beyond saying that the PCM on any modern vehicle, including modern Ford vehicles, consists of a collection of microprocessors, each of which runs on its own programming to perform different functions. These functions include, but are not limited to, throttle control, ignition and valve timing control, and fuel pressure control, among many others.
However, it is important to point out that while each microprocessor has its own programming and communication protocols, the programming of each microprocessor must be compatible with a) each other, and the larger operating system that facilitates effective communication between all the microprocessors contained within the PCM. To explain this further, we can use a modern PC as an analogy.
For our hypothetical computer to work as expected, the pieces of programming within it that control the display, sound playback, and keyboard inputs must be compatible not only with each other and a computer operating system but also with all of the dozens of background processes and drivers that “bind” the larger programming architecture into a cohesive structure that allows one to say, watch videos by making the appropriate keyboard and other inputs.
Similarly, when the PCM on say, a new Ford Taurus SHO is first programmed in the factory, the relevant and correct programming is loaded into each microprocessor, as well as into the operating system that manages and oversees the operation of each microprocessor. More importantly, though, just like any operating system which has gone through several iterations, the specific version, or calibration version, of a new PCM’s programming is saved in a dedicated file within the PCM.
In practice, a calibration version number refers not only to the version of the software within the PCM, but also all patches, bug fixes, updates, and other information. So, in a fully functional PCM, all the microprocessors in the unit work together seamlessly to control the engine management system, fuel delivery system, and other functions like the ABS, and the interface between the engine and the transmission.
However, given the extreme complexity of a) modern PCM circuitry, and b) the complex communication systems within the PCM, it should be clear that the failure of one microprocessor or communication path could, and does affect the overall operation of the PCM. Thus, if the PCM detects such a failure, it will recognize that it cannot control several critical engine management functions effectively (if at all), and it will set trouble code U0300, and illuminate a warning light.
See the section about “Causes” for more details on possible causes of trouble code U0300.