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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 Jimmy - 2WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 DTC U1000 Class 2 Communication Malfunction

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. When a module does not associate an identification number with at least one critical parameter within about five seconds of beginning serial data communication, DTC U1000 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

Conditions for Running the DTC
    • Voltage supplied to the module is in the normal operating voltage range (approximately 9 - 16 volts).
    • Diagnostic trouble codes U1300 and U1301 do not have a current status.
    • The vehicle power mode (ignition switch position) 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 five 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 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 U1000 will clear upon receipt of a scan tool Clear DTCs command.

Diagnostic Aids

When a malfunction (such as a blown fuse to a module) occurs while modules are communicating, a Lost Communication DTC is set current. When the modules stop communicating (ignition is turned off) the current Lost Communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the blown fuse will not be learned by the other modules so U1000 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 is set.

Test Description

  1. A Lost Communication with 1XXX DTC with a history status may indicate the cause of U1000.

    2.

  2. The module(s) which is not communicating is the likely cause of U1000 being set.

    3.

  3. The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set Lost Communication with 1XXX DTCs for those modules that it was monitoring.

    4.

  4. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.

    5.

  5. If all modules are communicating, the module which set U1000 may have done so due to some other condition.

    6.

  6. The module which set U1000 is the likely cause of the malfunction.

Step

Action

Yes

No

1

Did you record any DTCs in the range of U1001-U1199 with a history status?

Go to DTC List

Go to Step 2

2
  1. Turn on the ignition leaving 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 schematic showing which modules are connected to the class 2 serial data circuit.

Does any module(s) on the class 2 serial data circuit not communicate?

Go to Step 3

Go to Step 11

3
  1. Disconnect the star connector.
  2. Inspect for poor connections/terminal tension at the star connector. Refer to Connector Repairs .

Did you find and correct the condition?

Go to Step 7

Go to Step 4

4

Test the class 2 serial data circuit of the module that is not communicating for an open. Refer to Wiring Repairs .

Did you find and correct the condition?

Go to Step 7

Go to Step 5

5

Inspect for poor connections/terminal tension at the class 2 serial data circuit of the module that is not communicating. Refer to Connector Repairs .

Did you find and correct the condition?

Go to Step 7

Go to Step 6

6

Replace the module which is not communicating. Refer to the appropriate Repair Instructions for module replacement.

Did you complete the replacement?

Go to Step 9

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7
  1. Install a scan tool.
  2. Turn on the ignition leaving 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 the applicable DTC table

Go to Step 8

8

Clear the DTCs using the scan tool.

Did you complete the action?

Go to Step 9

--

9

Select the Display DTCs function for the module(s) which had U1000 set as a current DTC.

Does the scan tool display DTCs which do not begin with a U?

Go to the applicable DTC table

Go to Step 10

10

Clear the DTCs using the scan tool.

Did you complete the action?

System OK

--

11

Did you record any other DTCs for the module(s) which had U1000 set as a current DTC?

Go to the applicable DTC table

Go to Step 12

12
  1. Install a scan tool.
  2. Turn on the ignition leaving the engine off.
  3. Clear the DTCs using the scan tool.
  4. Turn off the ignition for at least 5 seconds.
  5. Turn on the ignition leaving the engine off.
  6. Select the Display DTCs function.

Does the scan tool display U1000 set as a current DTC?

Go to Step 13

System OK

13

Replace the module which had U1000 set as a current DTC. Refer to the appropriate Repair Instructions for module replacement.

Did you complete the replacement?

System OK

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