What does it mean?
This code is concerned with the communications circuit between the engine control module and other control modules throughout the vehicle. This communications circuit is most often referred to as Controller Area Network bus communications, or simply put, CAN bus. Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to communicate with the engine control module, depending on which circuit is affected.
The CAN bus communication system is wired throughout the vehicle, however for this code we are going to concern ourselves with CAN C, or High Speed CAN, which may also be referred to as under-hood or under-vehicle CAN bus, as the modules that typically communicate on this network can be found under-hood or under-car. What is causing this code to set is one or more modules have the task of communicating with the ECM/PCM on CAN C to make sure that it is able to communicate or “check in”.
If unable to check in, one or more modules can set the U0100 code to indicate that at some point in time it lost the ability to talk to the ECM/PCM. This code is usually a “memory” code, or something that occurred in the past and may be nothing to worry about. If the code continues to come back, further checks will be needed to find out why the modules are losing communication with the ECM/PCM.