GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Montana 🢒 DTC 🢒 DTC U1000 and U1255

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 5 seconds of beginning serial data communication, DTC U1000 or U1255 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 to 16 volts.
    • Diagnostic trouble codes U1300 and U1301 and U1305 do not have a current status.
    • 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 or U1255 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 or U1255 will clear when the predetermined number of ignition cycles has occured without a malfunction.

Diagnostic Aids

When a malfunction, such as an open fuse to a module, occurs while modules are communicating, a Lost Communication DTC is set as a current DTC. When the modules stop communicating the current Lost Communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so DTC U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only DTC U1000 or U1255 is set.

Test Description

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

  1. A Lost Communication with XXX DTC with a history status may indicate the cause of U1000 or U1255.

    2.

  2. The modules which are not communicating are the likely cause of U1000 or U1255 being set. The following modules communicate on the class 2 serial data circuit:

    3. • DIC
    • EBCM
    • IPC
    • MSM
    • PCM
    • PK3
    • Radio
    • SDM
    • VIU, if equipped

  3. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.

    4.

  4. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.

    5.

  5. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.

    6.

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

    7.

  7. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.

    8.

  8. The module which set U1000 or U1255 is the likely cause of the malfunction.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

1

Did you record any DTCs in the range of U1001-U1254 with a history status?

Go to DTC U1001-U1254

Go to Step 2

2

  1. Turn ignition ON, 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.

Are there any modules on the class 2 serial data circuit not communicating?

Go to Step 3

Go to Step 13

3

Test the battery positive voltage circuits and the ignition voltage circuits of the module that is not communicating for an open or short to ground. Refer to Control Module References in Body Control System for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 4

4
  1. Turn ignition OFF.
  2. Test the ground 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 9

Go to Step 5

5

  1. Disconnect the star connector.
  2. Inspect for poor connection at the star connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6

Test the class 2 serial data circuit 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 9

Go to Step 7

7

Inspect for poor connections at the battery positive voltage circuits, the ignition voltage circuits, the ground circuits, and the class 2 serial data circuit of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

Replace the module which is not communicating. Refer to Control Module References in Body Control System for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 11

--

9
  1. Install a scan tool.
  2. Turn ignition ON, engine OFF.
  3. Select the Display DTCs function for the module which was not communicating.

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

Go to Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 10

10

Use the scan tool in order to clear the DTCs.

Did you complete the action?

Go to Step 11

--

11

Select the Display DTCs function for the modules which had U1000 or U1255 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 the applicable Diagnostic System Check

Go to Step 12

12

Use the scan tool in order to clear the DTCs.

Did you complete the action?

System OK

--

13

Did you record any other DTCs for the modules which had U1000 or U1255 set as a current DTC?

Go to Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 14

14

  1. Install a scan tool.
  2. Turn ignition ON, engine OFF.
  3. Use the scan tool in order to clear the DTCs.
  4. Turn ignition OFF, wait at least 5 seconds.
  5. Turn ignition ON, engine OFF.
  6. Select the Display DTCs function.

Does the scan tool display U1000 or U1255 set as a current DTC?

Go to Step 15

Go to Diagnostic Aids

15

Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References in Body Control System for the appropriate repair instructions.

Did you complete the replacement?

System OK

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