GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Safari - 2WD 🢒 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 five 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 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.

Conditions for Running the DTC
    •  Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
    •  DTCs U1300, U1301 or U1305 do not have a current status.
    •  The vehicle power mode requires serial data communication to occur.

Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

Conditions for Clearing the DTC
    • A current DTC clears when the malfunction is no longer present.
    • A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

Diagnostic Aids

When a malfunction 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 U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only 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.

  1. The module which are not communicating will be the likely cause of U1000 or U1255 being set. For a list of the modules that communicate on the class 2 serial data circuit, refer to Data Link Communications Description and Operation

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

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

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

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

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

  1. 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 DTC U1001-U1254

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.
  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. Refer to Data Link Communications Description and Operation .

Are there any module on the class 2 circuit that do not communicate?

Go to Step 3

Go to Step 13

3

Test the following circuits of the module that is not communicating for an open or a short to ground.

    • The battery voltage
    • The ignition inputs
    • The ignition outputs
        Refer to the following:
       -  Control Module References in body control systems for the applicable schematic
       -  Circuit Testing in Wiring Systems
       -  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 9

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:
    3. •  Control Module References in body control systems for the applicable schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 9

Go to Step 5

5

  1. Remove the comb from the star connector.
  2. Inspect for poor connections at the star connector. Test for poor connections of the comb. 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 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 following circuits of the module that is not communicating:

    • The battery voltage
    • The ignition inputs
    • The ignition outputs
    • The class 2
    • The grounds
        Refer to the following:
       -  Control Module References in body control systems for the applicable schematic
       -  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 systems for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 11

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9
  1. Connect 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 Control Module References in body control systems for the applicable Diagnostic System Check

Go to Step 10

10

Use the scan tool in order to clear the DTCs.

Did you clear the DTCs?

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 systems 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 systems for the applicable Diagnostic System Check

Go to Step 14

14

  1. Connect a scan tool.
  2. Turn ON the ignition leaving 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 leaving the 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 systems for the appropriate repair instructions.

Did you complete the replacement?

System OK

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