GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Kodiak C-Series Motorhome (Cutaway) C4/C5 🢒 Restraints 🢒 SIR 🢒 DTC U0073

Circuit Description

The engine control module (ECM), electronic brake control module (EBCM) if applicable, fuel injector control module (FICM) if applicable, and transmission control module (TCM) if applicable, communicate via a controller area network (CAN) serial data bus. Modules connected to the CAN serial data circuits monitor communications. During normal vehicle operation, information and commands are exchanged among modules. A no communication condition may result from a short to voltage, short to ground, an open in the CAN High or Low circuits, or short between the CAN high and low circuits.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U0073 Loss of CAN Communications

Conditions for Running the DTC
    • The ignition switch is ON.
    • The module supply voltage is in the normal operating range of 9-16 volts.

Conditions for Setting the DTC

A no communication condition is detected on CAN serial data circuits.

Action Taken When the DTC Sets
    • The module uses a default value for the missing parameter.
    • The malfunction indicator lamp (MIL) illumination depends on DTC type. For a detailed description about DTCs types refer to Diagnostic Trouble Code (DTC) Types for the 6.6L (LLY) engine. For information about DTC type definitions refer to Diagnostic Trouble Code (DTC) Type Definitions for the 6.6L (LLY) engine.

Conditions for Clearing the MIL/DTC

MIL or DTC clearing depends on DTC type. For a detailed description about DTCs types refer to Diagnostic Trouble Code (DTC) Types for the 6.6L (LLY) engine. For information about DTC type definitions refer to Diagnostic Trouble Code (DTC) Type Definitions for the 6.6L (LLY) engine.

    • DTC Type C
    • A current DTC and MIL clear 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

The modules communicating on the CAN serial data circuits are wired parallel to each other until the respective circuits are spliced together outside of the ECM. An open in the CAN circuits for one module will not affect other modules. A short to ground or short to voltage affects all modules no matter where the failure occurs.

Test Description

The number below refers to the step number on the diagnostic table.

  1. This test isolates a shorted FICM from a shorted TCM, ECM, or EBCM. By reading the signal from the FICM after the wiring is tested, without a TCM connected, an FICM signal generating problem can be diagnosed. When the CAN circuits are open between two components, both components generate a 30,000-40,000 hertz signal. When the system is not open, the frequency is 3,000-3,500 hertz.

Step

Action

Values

Yes

No

Schematic Reference: Data Link Connector Schematics

Connector End View Reference: Computer/Integrating Systems Connector End Views

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

2

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

  1. Turn OFF the ignition.
  2. Disconnect the engine control module (ECM), transmission control module (TCM) if applicable, electronic brake control module (EBCM) if applicable, and fuel injector control module (FICM) if applicable harness connectors which contain controller area network (CAN) circuits.
  3. Test the CAN serial data circuits for a short to ground, short to voltage, an open in the CAN High circuit, an open in the CAN Low circuit and a short between the CAN High and CAN Low circuits. Refer to Circuit Testing .

Did you find and correct the condition?

--

Go to Step 14

Go to Step 3

3

Important: If the vehicle is not equipped with an fuel injector control module (FICM), answer YES to this step.

  1. Reconnect the FICM.
  2. Remove the connector pins for the CAN circuits from the ECM harness connectors.
  3. Reconnect the ECM connectors.
  4. Start the engine.
  5. Measure the frequency across the CAN circuits in the harness with a DMM.

Does the frequency measure more than the specified value?

35,000 Hz

Go to Step 4

Go to Step 7

4

Important: If the vehicle is not equipped with a TCM, answer YES to this step.

  1. Turn OFF the ignition.
  2. Reconnect the TCM, if equipped.
  3. Clear any DTCs in memory.
  4. Start the engine.
  5. Measure the frequency across the CAN circuits in the harness with a DMM.

Does the frequency measure more than the specified value?

35,000 Hz

Go to Step 5

Go to Step 8

5

Important: If the vehicle is not equipped with an EBCM, answer YES to this step.

  1. Turn OFF the ignition.
  2. Reconnect the EBCM, if equipped.
  3. Clear any DTCs in memory.
  4. Start the engine.
  5. Measure the frequency across the CAN circuits in the harness with a DMM.

Does the frequency measure more than the specified value?

35,000 Hz

Go to Step 6

Go to Step 9

6

Inspect for an intermittent or poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 14

Go to Step 10

7

Inspect for an intermittent or poor connection at the FICM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you fine and correct the condition?

--

Go to Step 14

Go to Step 11

8

Inspect for an intermittent or poor connection at the TCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you fine and correct the condition?

--

Go to Step 14

Go to Step 12

9

Inspect for an intermittent or poor connection at the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you fine and correct the condition?

--

Go to Step 14

Go to Step 13

10

Replace the ECM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 14

--

11

Replace the FICM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 14

--

12

Replace the TCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 14

--

13

Replace the EBCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 14

--

14
  1. Clear the DTCs with a scan tool.
  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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 15

15

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK