GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC C Sierra - 2WD 🢒 Body 🢒 Data Link Communications 🢒 DTC U0073

Circuit Description

Modules connected to the controller area network (CAN) serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. If no communication is detected on the CAN serial data circuit, the DTC is set. A total non-communication condition may be caused by a short to battery or voltage on any of the CAN circuits or a short between the CAN high and low circuits.

The following modules communicate on CAN serial data (LLY engine):

    • The engine control module (ECM)
    • The transmission control module (TCM)
    • The fuel injector control module (FICM)
    • The glow plug control module (GPCM)

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U0073 High Speed CAN Communication Bus

Conditions for Running the DTCs
    • Voltage supplied to the modules is in the normal operating voltage range.
    • The engine is running.

Conditions for Setting the DTC

A total non-communication condition is detected on the CAN serial data circuit.

Action Taken When the DTC Sets
    • The module uses a default value for the missing parameter.
    • The malfunction indicator lamp (MIL) will illuminate on the second consecutive ignition cycle that the diagnostic runs and fails, for the type B DTCs. For a detailed description about DTC types, refer to Diagnostic Trouble Code (DTC) Type Definitions in Engine Controls - 6.6L (LLY).

Conditions for Clearing the MIL/DTC

The MIL/DTC clearing depends on DTC type. For information about DTC type, refer to Diagnostic Trouble Code (DTC) Types in Engine Controls - 6.6L (LLY):

DTC Type B

    • The MIL clears after 3 ignition cycles when the malfunction is no longer present.
    • 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.

DTC Type C

    • 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.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

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

1
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

Go to Step 2

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

2
  1. Turn OFF the ignition.
  2. Disconnect the fuel injector control module (FICM).
  3. Disconnect the CAN circuits from the FICM.
  4. Reconnect the FICM.
  5. Operate the vehicle within the Conditions for Running the DTC.
  6. Retrieve DTCs from engine control module (ECM) and transmission control module (TCM).

Does the DTC reset as current in any module?

Go to Step 3

Go to Step 6

3
  1. Turn OFF the ignition.
  2. Disconnect the glow plug control module (GPCM).
  3. Disconnect the CAN circuits from the GPCM.
  4. Reconnect the GPCM.
  5. Operate the vehicle within the Conditions for Running the DTC.
  6. Retrieve DTCs from ECM and TCM.

Does the DTC reset as current in any module?

Go to Step 4

Go to Step 6

4

Important: 

   • If the vehicle is not equipped with TCM skip to the next step.
   • If the code set only in TCM, disconnect the ECM in this step.
   • If the code was set by ECM or by both modules, disconnect the TCM in this step.

  1. Turn OFF the ignition.
  2. Disconnect the module conform to the important statement.
  3. Disconnect the CAN circuits from the module.
  4. Reconnect the module.
  5. Operate the vehicle within the Conditions for Running the DTC.
  6. Retrieve DTCs from the module still connected.

Does the DTC reset as current?

Go to Step 5

Go to Step 6

5

Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use DMM MIN/MAX function to capture intermittent conditions.

  1. Turn OFF the ignition.
  2. Disconnect the last module still connected to CAN bus.
  3. Test the CAN serial data circuits for a short to ground, short to voltage, an open, or a short between them. Refer to the following in Wiring Systems:

Did you find and correct the condition?

Go to Step 8

Go to Step 6

6

Inspect for an intermittent or poor connections at the last disconnected/reconnected module. 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 8

Go to Step 7

7

Important:: Perform the module setup procedure if required.

Replace the last disconnected/reconnected module. Refer to Control Module References in Body Control System.

Did you complete the replacement?

Go to Step 8

--

8
  1. Reconnect all wires and modules.
  2. Clear the DTCs with a scan tool.
  3. Turn OFF the ignition for 30 seconds.
  4. Start the engine.
  5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

System OK.