While the vehicle is operating normally, the control modules are connected via the Engine General Local Area Network (GMLAN). Its function is to monitor the output data from all the control modules that are related to the operation of the vehicle. Then, this data is compared to the network of computers that monitor the operating systems in an automated manufacturing environment.
The GMLAN system is like a clearinghouse for all the information exchanged between the vehicle modules. This system also monitors the communication of the modules, which contains information about the signals to be exchanged between them.
The U0121 OBDII diagnostic code is set when communication between the ABS Brake Control Module and the PCM is lost for a period. In most cases, it occurs after a communication interruption of 250 milliseconds (0.25 sec).