Do you know what a serial data network is ? U codes are problems associated the data buss . A module isn't communicating or the buss wiring is shorted etc.....
Data Link Communications Description and Operation
Do you know what an how many modules are on your vehicle ?
The communication among control modules is performed primarily through the GMLAN high speed serial data circuit and the GMLAN low speed serial data circuits. The modules that need real time communication are attached to the high speed GMLAN network. The body control module (BCM) is the serial data gateway between the networks. The purpose of the gateway is to translate serial data messages between the GMLAN high speed buss and the GMLAN low speed buss. The controller area network (CAN) is another serial data communication network used on this vehicle which is dedicated to the electronic brake control module (EBCM) subsystem for stabilization. Below are more detailed descriptions of the individual networks. The gateway will interact with each network according to that network's transmission protocol. Refer to Body Control System Description and Operation for more information about the gateway.
Your best bet would be to take your vehicle to a qualified repair shop .
DTC U2100 00: Controller Area Network (CAN) Bus Communication
The GMLAN serial data circuits are controller area network (CAN) serial data buses used to communicate information between the control modules. Typical data transmission speeds must be high enough to ensure that a required real-time response is maintained. On this vehicle there are 2 totally different types of GMLAN serial data circuits, the high speed 2-wire circuit and low speed single wire circuit. Refer to Data Link Communications Description and Operation for the complete description of these buses. The GMLAN serial data circuits also communicate directly to the DLC connector. Messages are interpreted by the externally connected CANdi module which acts as a transceiver for the scan tool.
Modules connected to the GMLAN serial data circuits monitor communications during normal vehicle operation, where operation information is exchanged among the modules. All the modules on the GMLAN networks, maintain a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error-free messages. If the TEC value exceeds 255, the controller removes itself from the network and DTC U2100 will result.
016 CAN Communications