GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 GMC T-Series Medium Duty Truck F6/F7/F8 🢒 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 ID number of the module which sent the message for State of Health monitoring, node alive messages. A critical operating parameter is one which, when not received, requires that the module use a default value for the parameter. When a module does not associate an ID number with at least one critical parameter within about 5.5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an ID number associated with it, the DTC will only be reported once.

The table below lists the modules on the class 2 serial data circuit that will set a U1000 or U1255 and should actively communicate the previously described messages on the class 2 serial data circuit.

Control Module

ID Number

Body Control Module (BCM)

064

Electronic Brake Control Module (EBCM)

040

Instrument Panel Cluster (IPC)

096

Powertrain Control Module (PCM)

016

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC U1000 Class 2 Data Link
    • DTC U1255 Lost Communications

Conditions for Running the DTC
    • The ignition switch run power mode is selected.
    • The modules voltage supply is within normal operating value, approximately 9-16 volts.

Conditions for Setting the DTC

At least one critical operating parameter has not been associated with a modules assigned ID number within about 5.5 seconds after beginning serial data communication.

Action Taken When the DTC Sets
    • The malfunction indicator lamp (MIL) illuminates.
    • The module will use default values for the missing parameters.

Conditions for Clearing the DTC
    • A current DTC will clear when all critical operating parameters for the module have been associated with an ID number, or at the end of the current ignition cycle.
    • 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 such as a open fuse to a module occurs while modules are communicating, a loss of XXX communication DTC is set as current. When the modules stop communicating or the ignition is turned OFF, the current loss of XXX communication DTC is cleared but the history DTC remains. When the modules begin to communicate again or the ignition is turned ON, 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 DTC U1001-U1254, lost communications with XXX, with a history status may indicate the cause of U1000 or U1255.

    2.

  2. The modules not communicating are the likely cause of U1000 or U1255. The modules available on the class 2 serial data circuit are listed in the Circuit Description.

    3.

  3. 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.

  4. 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.

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

    6.

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

Step

Action

Yes

No

Schematic Reference: Data Communication Schematics

Connector End View Reference: Data Communication Connector End Views

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?

Diagnostic Trouble Code (DTC) List - Vehicle

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 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:

    •  Circuit Testing
    •  Wiring Repairs

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 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:

   •  Connector Repairs

  1. Turn OFF the ignition.
  2. Test the class 2 serial data circuits of the module 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 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:

    •  Connector Repairs

Did you find and correct the condition?

Go to Step 8

Go to Step 7

7

Replace the module that is not communicating. Refer to Control Module References for replacement, setup, and programming.

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 Diagnostic System Check - Vehicle

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 or U1255 set as a current DTC.

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

Go to Diagnostic System Check - Vehicle

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 or U1255 set as a current DTC?

Go to Diagnostic System Check - Vehicle

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 or U1255 set as a current DTC?

Go to Step 14

Go to Diagnostic Aids

14

Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

System OK

--