Some control modules are connected to the CAN bus (Control Area Network). This bus maintains constant information on vehicle operating data and commands. These modules contain pre-recorded information related to the messages that need to be exchanged between the serial data circuits.
On the other hand, these messages are monitored, and also some periodic messages are used from the receiver module. These act as availability indicators of the transmitter module. Typically, the supervision timeout is 250 ms, and each of them includes a transmitter module ID.
The U0020 OBDII fault code must be set when fewer modules are detected on the bus before they can learn the IDs of other controllers. This code indicates that a supervised periodic message, including the availability of the transmitter module, has not been received.