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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 528 (DATA LINK COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC U0100 TO U0299: GMLAN HIGH SPEED SERIAL DATA MISSINGTEST DESCRIPTION

1999 GMC Forward Control 5.7 R, Automatic

Test Description

1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Description

WARNING: This page is about a different car, the 2004 Pontiac Aztek and 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

1: A total loss of communication on the GMLAN High Speed serial data link should be diagnosed first.
8: The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.

DTC U0100-U0299

Step	Action	Yes	No
Schematic Reference: Data Link Connector (DLC) Schematics Connector End View Reference: Data Link Communications Connector End Views
1	Install a scan tool. Start the engine. Retrieve the DTCs from the engine control module (ECM). Is DTC U0073 set as current?	Go to DTC U0001	Go to Step 2
2	IMPORTANT: Use the control module ID number list in order to determine which module is not communicating on GMLAN High Speed serial data bus. Turn ON the ignition, with the engine OFF. Test the following circuits of the module that is not communicating on GMLAN High Speed serial data bus for an open or a short to ground: The battery positive voltage input circuits The battery positive voltage output circuits The ignition voltage input circuits The ignition voltage output circuits The switched battery positive voltage circuits Refer to the following: Control Module References for the applicable schematic Circuit Testing in Wiring Systems Wiring Repairs in Wiring Systems Did you find and correct the condition?	Go to Step 7	Go to Step 3
3	Turn OFF the ignition. Test the ground circuits of the module that is not communicating on GMLAN High Speed serial data bus for an open. Refer to the following: Control Module References for the applicable schematic Circuit Testing in Wiring Systems Wiring Repairs in Wiring Systems Did you find and correct the condition?	Go to Step 7	Go to Step 4
4	Test the GMLAN High Speed serial data circuits of the module that is not communicating on GMLAN High Speed serial data bus for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	Go to Step 7	Go to Step 5
5	Inspect the harness connectors of the module that is not communicating on GMLAN High Speed serial data bus for poor connections and terminal tension at the following circuits: The battery positive voltage input circuits The battery positive voltage output circuits The ignition voltage input circuits The ignition voltage output circuits The switched battery positive voltage supply circuits The ground circuits The GMLAN High Speed serial data circuits Refer to the following: Control Module References for the applicable schematic Testing for Intermittent Conditions and Poor Connections in Wiring Systems Connector Repairs in Wiring Systems	Go to Step 7	Go to Step 6
6	IMPORTANT: Perform the programming or setup procedure for the module, if required. Replace the module that is not communicating on GMLAN High Speed serial data bus. Refer to Control Module References for the applicable replacement procedure.Did you complete the replacement?	Go to Step 7	-
7	Select the Display DTCs function for the modules which had the Loss of Communications DTC set. Does the scan tool display any DTCs which do not begin with a "U"?	Go to Control Module References	Go to Step 8
8	Use the scan tool in order to clear the DTCs. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared. Did you complete the action?	Go to Control Module References	-

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.
• Brakes feel soft, pull hard to one side, or make grinding noises.
• The engine overheats, stalls repeatedly, or misfires under load.
• You are missing required tools, torque specs, or safe lifting equipment.
• You are not confident in the next step or safety outcome.