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HOMESERVICE MANUALSCHEVROLET1998SUBURBAN K2500, 7.4 J, PART TIMEREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 604 (DATA LINK COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC U1001 TO U1254: MODULE U1XXX COMMUNICATIONTEST DESCRIPTION

1998 Chevrolet Suburban K2500, 7.4 J, Part Time

Test Description

1998 Chevrolet Suburban K2500, 7.4 J, Part TimeSECTION 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 number below refers to the step number on the diagnostic table.

6: The module which was not communicating on the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.

DTC U1001-U1254

Step	Action	Yes	No
Schematic Reference: Data Link Connector (DLC) Schematics Connector End View Reference: Master Electrical Component List and Inline Harness Connector End Views in Wiring Systems
1	IMPORTANT: Use the control module ID number list in order to determine which module is not communicating. Test the following circuits of the module that is not communicating 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 in Body Control System for the applicable schematic Circuit Testing in Wiring Systems Wiring Repairs in Wiring Systems Did you find and correct the condition?	Go to Step 6	Go to Step 2
2	Turn OFF the ignition. Test the ground circuits of the module that is not communicating for an open. Refer to the following: Control Module References in Body Control System for the applicable Schematic Circuit Testing in Wiring Systems Wiring Repairs in Wiring Systems Did you find and correct the condition?	Go to Step 6	Go to Step 3
3	Test the class 2 serial data circuits of the module that is not communicating for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	Go to Step 6	Go to Step 4
4	Inspect the harness connectors of the module that is not communicating 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 class 2 serial data circuits Refer to the following: Control Module References in Body Control System for the applicable schematic Testing for Intermittent Conditions and Poor Connections in Wiring Systems Connector Repairs in Wiring Systems Did you find and correct the condition?	Go to Step 6	Go to Step 5
5	IMPORTANT: Perform the programming or set up procedure for the module, if required. Replace the module that is not communicating. Refer to Control Module References for the applicable replacement procedure.Did you complete the replacement?	Go to Step 6	-
6	Install a scan tool. Turn ON the ignition with the engine OFF. Select the Display DTCs function for the module which was not communicating. Does the scan tool display any DTCs which do not begin with a "U"?	Go to Control Module References for the applicable Diagnostic System Check	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 for the applicable Diagnostic System Check	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 for the applicable Diagnostic System Check	-

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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.