For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC U1000: Class 2 Data Link
DTC U1255: Loss of Class 2 Communication

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage
--- | --- | --- | ---
Battery Positive Voltage Circuits | U0100-U0299 | U0100-U0299 | --
Ignition Voltage Circuits | U0100-U0299 | U0100-U0299 | --
Module Ground Circuits | -- | U0100-U0299 | --

### 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 (SOH) monitoring. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within about 5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. If more than one critical parameter does not have an identification number association, the DTC will only be reported once.

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.

• | Diagnostic trouble codes U1300, U1301, and U1305 do not have a current status.

• | The module operation or the vehicle power mode requires serial data communication to occur.

### Conditions for Setting the DTC

At least one critical operating parameter has not been associated with an identification number within about 5 seconds after beginning serial data communication.

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### Conditions for Clearing the DTC

• | A current DTC U1000 will clear when all critical operating parameters for the module have been associated with an identification number, or at the end of the current ignition cycle.

• | A history DTC U1000 will clear after 50 ignition switch ON/OFF cycles with no repeat of the failure.

### Diagnostic Aids

If a malfunction such as an open fuse to a module occurs while serial data communication is active, a Lost Communication DTC for the specific module is set as a current DTC in other modules. When all serial data communication becomes inactive, the current Lost Communication DTC is cleared, but the history DTC remains. When serial data communication resumes, the module with the open fuse will not be learned by the other modules, so DTC U1000 or U1255 is set as current by the other modules.

In the event of a malfunction occurrence while serial data communication is inactive, DTC U1000 or U1255 is set without a loss of communication DTC for the specific module.

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Verify that no DTC within the range of U1001-U1254 exists as current or history in any module.

⇒ | If any DTC within the range of U1001-U1254 does exist, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

2. Ignition OFF, disconnect the harness connector of the module that is not communicating.
3. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.

4. Verify that a test lamp illuminates between all ignition voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.

5. Test for less than 1 ohm between the module ground circuits and ground.

⇒ | If greater than the specified range, test the ground circuit for open/high resistance.

6. Test for less than 1 ohm in the class 2 serial data circuit(s) between the non-communicating module and the module that displays the DTC.

⇒ | If greater than the specified range, test the serial data circuit(s) for open/high resistance.

7. If all circuits test normal, replace the module that is not communicating.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | GMLAN Wiring Repairs

• | Control Module References for module replacement, setup, and programming