U1064

DTC U1001-U1254

CIRCUIT DESCRIPTION
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and 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. 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 communications malfunction is required to send a Node Alive message every 2 seconds. When no message is detected from a learned identification number for 5-10 seconds, a DTC U1xxx where xxx is equal to the 3-digit identification number is set.

Class 2 Control Module ID:

The control module ID number list below provides a method for determining which module is not communicating. A module with a class 2 serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The modules that can communicate will set a DTC indicating the module that can not communicate. The following modules communicate on class 2 serial data circuit:

The BCM emulates some devices that are on high speed GMLAN serial data circuit to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of those high speed GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuit and are emulated by BCM on class 2 serial data circuit:

GMLAN High Speed Control Module ID:

When more than one Loss of Communication DTC is set in either one module or in several modules, diagnose the DTCs in the following order:
1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
2. The DTC which is reported the most times.
3. From the lowest number DTC to the highest number DTC.

CONDITIONS FOR RUNNING THE DTC

Failure Type Byte (FTB):

- The following diagnostic trouble codes do not have a current status:
- U1300
- U1301
- U1305
- The vehicle power mode requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
A node alive message has not been received from a module with a learned identification number within the last 5-10 seconds.

ACTION TAKEN WHEN THE DTC SETS
The module uses a default value for the missing parameter.

CONDITIONS FOR CLEARING THE DTC
- A current DTC clears when the malfunction is no longer present.
- A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

DIAGNOSTIC AIDS
- An intermittent open between the inoperative module and a star connector may cause these codes.
- A poor connection at the inoperative module or a star connector may cause these codes.
- An intermittent open in a star connector may cause these codes.
- An improperly powered module may cause these codes.
- An improperly communication on the high speed GMLAN serial data circuit may cause these codes.

TEST DESCRIPTION

Step 1 - Step 9:

Step 10 - Step 12:

The numbers below refer to the step numbers on the diagnostic table.
2. A module that does not communicate in high speed GMLAN link may cause the setting of the class 2 DTCs.
3. A module which loses power during an ignition cycle will cause other modules to set lost communication DTCs.
4. A module which loses power during an ignition cycle will cause other modules to set lost communication DTCs.
5. The malfunction is due to an open in the class 2 serial data circuit or an open in the module.
9. The module which was not communicating may have set lost communication DTCs for those modules that it was monitoring.
11. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.