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Test Description

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. 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. 1:  A DTC U1001-U1254 Lost Communications with XXX with a history status may indicate the cause of U1000.
  2. 2:  The modules not communicating are the likely cause of U1000. The modules that are available on the class 2 serial data circuit are listed in the Circuit Description.
  3. 6:  The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set DTC U1001-U1254 Lost Communications with XXX for those modules that it was monitoring.
  4. 9:  The modules which can communicate indicate the module which cannot communicate. You must clear the serial data communication DTCs from these modules to avoid future misdiagnosis.
  5. 12:  If all modules are communicating, the module which set U1000 may have done so due to some other condition.
  6. 14:  The module which set U1000 is the likely cause of the malfunction.
DTC U1000

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 
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Record the DTCs set in by each module. If using a Tech 2, use the Class 2 DTC Check feature to determine which modules have DTCs set.
Did you record any DTCs in the range of U1001-U1254 with a history status?		 Go to Diagnostic Trouble Code (DTC) List  Go to Step 2 
2 
  1. Turn ON the ignition, with the engine OFF.
  2. Attempt to communicate with each module on the class 2 serial data circuit. If using a Tech 2, obtain this information using the class 2 Message Monitor feature.
  3. Record all of the modules communicating on the class 2 serial data circuit.
  4. Compare the list of modules which are communicating to the list given in Circuit Description.
Does any module on the class 2 serial data circuit not communicate?		 Go to Step 3  Go to Step 12 
3 Test the following circuits of the module that is not communicating for an open or short to ground:
  • The battery positive voltage supply circuits
  • The switched battery positive supply circuits
  • The battery positive voltage output circuits
  • The ignition voltage input circuits
  • The ignition voltage output circuits

Refer to the following:
Did you find and correct the condition?		 Go to Step 8  Go to Step 4 
4
  1. Turn OFF the ignition.
  2. Test the ground circuits of the module that is not communicating for an open. Refer to the following:
Did you find and correct the condition?		 Go to Step 8  Go to Step 5 
5
IMPORTANT: Inspect all connectors in the serial data communications circuit.
Refer to the following:
  1. Turn OFF the ignition.
  2. Test the class 2 serial data circuits of the module that is not communicating for an open. Refer to the following:
Did you find and correct the condition?		 Go to Step 8  Go to Step 6 
6  Inspect the harness connector of the module that is not communicating for poor connections and terminal tension at the following circuits:
  • The battery positive voltage input circuits
  • The switched battery positive voltage supply
  • The battery positive voltage output circuits
  • The ignition voltage input circuits
  • The ignition voltage output circuits
  • The ground circuits
  • The class 2 serial data circuits

Refer to the following:
Did you find and correct the condition?		 Go to Step 8  Go to Step 7 
7
IMPORTANT: Perform the module programming or setup procedure if required.
Replace the module that is not communicating. Refer to Control Module References in Body Control System for the appropriate Repair Instructions for the module replacement.Did you complete the replacement?		 Go to Step 10  -
8
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. 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 in Body Control System for applicable Diagnostic System Check Go to Step 9 
9  Use the scan tool in order to clear the DTCs.
Did you complete the action?		 Go to Step 10  -
10 Select the Display DTCs function for the modules which had U1000 set as a current DTC.
Does the scan tool display DTCs which do not begin with a "U"?		 Go to Control Module References in Body Control System for applicable Diagnostic System Check Go to Step 11 
11 Use the scan tool in order to clear the DTCs.
Did you complete the action?		 System OK -
12  Did you record any other DTCs for the modules which had U1000 set as a current DTC? Go to Control Module References in Body Control System for applicable Diagnostic System Check Go to Step 13 
13
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Use the scan tool in order to clear the DTCs.
  4. Turn OFF the ignition for at least 5 seconds.
  5. Turn ON the ignition with the engine OFF.
  6. Select the Display DTCs function.
Does the scan tool display U1000 set as a current DTC?		 Go to Step 14  Go to Diagnostic Aids
14 
IMPORTANT: Perform the module programming or setup procedure if required.
Replace the module which had U1000 set as a current DTC. Refer to Control Module References in Body Control System for the appropriate Repair Instructions for the module replacement.Did you complete the replacement?		 System OK -
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