Modules connected to the class 2 serial data circuit monitor for serial data communication during normal vehicle operation. Operation commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring, Node Alive messages. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that module's Node Alive message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communication malfunction is required to send a Node Alive message every 2-5 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1XXX (where XXX is equal to the three digit identification number of the inoperative module is set.

The Control Module ID Number list provides a method for determining which module is not communicating. A module with an internal class 2 serial data circuit malfunction of which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. Use the Control Module ID Number list in order to determine which module is not communicating and the DTC U1001- U1254 diagnostic table in order to diagnose the malfunction. As long as DTC U1300,U1301 and U1305 are not set, DTC U1001-U1254 will set.