GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Equinox 🢒 Brakes 🢒 Park Brake 🢒 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 5-10 seconds, a DTC U1xxx where xxx is equal to the 3-digit identification number is set.

The control module ID number list below provides a method for determining which module is not communicating. A module with a class 2 serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The modules that can communicate will set a DTC indicating the module that can not communicate. The following modules communicate on class 2 serial data circuit:

Control Module

ID Number

Power Steering Control Module (PSCM)

048

Body Control Module (BCM)

064

Inflatable Restraint Sensing and Diagnostic Module (SDM)

088

Instrument Panel Cluster (IPC)

096

Radio

128

Digital Radio Receiver (DRR)

137

Vehicle Communication Interface Module (VCIM)

151

The BCM emulates the control modules that are on high speed GMLAN serial data circuit to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of those high speed GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuit and are emulated by BCM on class 2 serial data circuit:

Control Module

ID Number

Engine Control Module (ECM)

017

Transmission Control Module (TCM)

024

Electronic Brake Control Module (EBCM)

040

When more than one Loss of Communication DTC is set in either 1 module or in several modules, diagnose the DTCs in the following order:

  1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
  2. The DTC which is reported the most times.
  3. From the lowest number DTC to the highest number DTC.

Conditions for Running the DTC
    • The following diagnostic trouble codes do not have a current status:
       - U1300
       - U1301
       - U1305
    • The vehicle power mode requires serial data communication to occur.

Conditions for Setting the DTC

A node alive message has not been received from a module with a learned identification number within the last 5-10 seconds.

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
    • An intermittent open between the inoperative module and a star connector may cause these codes.
    • A poor connection at the inoperative module or a star connector may cause these codes.
    • An intermittent open in a star connector may cause these codes.
    • An improperly powered module may cause these codes.
    • An improperly communication on the high speed GMLAN serial data circuit may cause these codes.

Test Description

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

  1. A module that does not communicate in high speed GMLAN link may cause the setting of the class 2 DTCs.

    2.

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

    3.

  3. A module which loses power during an ignition cycle will cause other modules to set lost communication DTCs.

    4.

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

    5.

  5. The module which was not communicating may have set lost communication 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.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Record the DTCs set in by each module.

Did you record any DTCs in the range of U2105-U2177 or U0100-U0299?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 3

3

Important:: Use the control module ID number list in order to determine which module is not communicating.

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

    • The battery positive voltage circuits
    • The ignition voltage circuits
    • Battery positive voltage output circuits
    • Switched battery positive voltage circuits
    • Instrument panel lamps dimmer switch signal circuit, if the non-communicating module is BCM

Refer to the following:

    •  Control Module References for the applicable schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 11

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 for the applicable schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 11

Go to Step 5

5

  1. Disconnect the star connector that is connected to the module that is not communicating.
  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 9

Go to Step 6

6

Test the class 2 serial data 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 9

Go to Step 7

7

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

    • The battery positive voltage circuits
    • The ignition voltage circuits
    • Battery positive voltage output circuits
    • Switched battery positive voltage circuits
    • Ground circuits
    • Class 2 serial data circuit

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 for replacement, setup and programming.

Did you complete the replacement?

Go to Step 11

--

9

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Retrieve DTCs from the module which was not communicating.

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

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

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

Retrieve DTCs from 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 Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 12

12
  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

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