Circuit/System Description

Control modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules have programmed information about what messages are needed to be exchanged on the serial data circuits. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module.

DTC U0020 will set when fewer control modules are detected on the bus before the control modules can learn the other control modules IDs. If the communication bus is opened, or if the control modules power supply is lost when the vehicle is in the OFF power mode, and then placed in the ON power mode, U0020 will set in specific control modules.