GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Sonoma Pickup - 2WD 🢒 Body and Accessories 🢒 Data Link Communications 🢒 DTC U1001-U1254

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. Once an identification number is learned by a module, it will monitor for that module's Node Alive message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a Node Alive message every 2 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx is set.

The Control Module ID Number list provides a method for determining which module is not communicating. A module with an internal class 2 serial data circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. The modules that can communicate will set a DTC indicating the module that cannot communicate. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx is set.

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.

Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past 5 seconds.

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.

Test Description

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

  1. A module which loses power during an ignition cycle will cause other modules to set Lost Communication DTCs.

    2.

  2. A module which loses power during an ignition cycle will cause other modules to set Lost Communication DTCs.

    3.

  3. The malfunction is due to an open in the class 2 serial data circuit or an open in the module.

    4.

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

    5.

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

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

1

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

    • Battery voltage
    • Ignition inputs
    • Ignition outputs

Refer to the following:

    •  Control Module References in Body Control System 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 2

2

  1. Turn OFF the ignition.
  2. Test the ground circuits of the module that is not communicating for an open.

Refer to the following:

    •  Control Module References in Body Control System 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 3

3

  1. Remove the comb from the star connector.
  2. Inspect for poor connections 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 7

Go to Step 4

4

Test the class 2 circuit of the module that is not communicating for an open between the module and the star connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 5

5

Inspect for poor connections at the following circuits of the module that is not communicating:

    • Battery voltage
    • Ignition inputs
    • Ignition outputs
    • Class 2
    • Grounds

Refer to the following:

    •  Control Module References in Body Control System for the applicable schematic
    •  Connector Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 7

Go to Step 6

6

Important: Replace and reprogram 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 9

--

7

  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 System for the applicable Diagnostic System Check

Go to Step 8

8

Use the scan tool in order to clear the DTCs.

Did you complete the action?

Go to Step 9

--

9

Select the Display DTCs function for the modules which had the Lost Communication with xxx DTC set.

Does the scan tool display any 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
  1. Use the scan tool in order to clear the DTCs.
  2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Did you complete the action?

System OK

--