For 1990-2009 cars

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.

Control Module | ID Number
--- | ---
Engine Control Module (ECM) | 016
Body Control Module (BCM) | 064
Sensing and Diagnostic Module (SDM) | 088
I/P Cluster | 096
OnStar® CIM | 151
Auto Climate Control Module | Auto Climate Control Module

### Diagnostic Aids

• | An intermittent open of the class 2 serial data circuit to the inoperative module may cause this code to set.

• | A poor connection at the inoperative module may cause this code to set.

• | An intermittent open in the splice may cause this code to set.

• | An improperly powered module may cause this code to set.

• | A current DTC will clear when a Node Alive (S.O.H) message from the failed identification number is detected on the class 2 serial data circuit or at the end of the current ignition cycle.

• | A history DTC will clear after 100 engine running cycles with no recurrence of the malfunction.

### DTC U1001 - U1254