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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2006 🢒 Solstice 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 DTC U0020

Circuit Description

Modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module.

DTC U0020 will set when fewer modules are detected on the buss before the modules can learn the other module's IDs. If the communication buss is opened, or if the a modules power supply is lost when the vehicle is in the OFF power mode, and then placed in the RUN power mode, U0020 will set in specific modules.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U0020 Low Speed CAN Communication Bus Performance

When more than one Low Speed CAN Communication Bus Performance DTC is set in either one 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.

This vehicle has DTCs which include DTC Symptoms. For more information on DTC Symptoms, refer to DTC Symptom Description .

DTC Symptom

DTC Symptom Descriptor

00

No Additional Information

Conditions for Running the DTCs
    • Voltage supplied to the modules is in the normal operating voltage range.
    • The vehicle power mode requires serial data communication to occur.
    • The DTC U0020 does not have a current status.

Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

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
    • A poor connection at the inoperative module may cause this code to set.
    • An improperly powered module may cause this code to set.

Test Description

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

  1. If body control module (BCM) will not power up the vehicle cannot establish the correct power mode. This step tests for the correct power supply conditions for the BCM.

    2.

  2. If DTC U0020 is set as history and you can communicate with the suspect module, the malfunction is an intermittent communication buss, power supply, ground or internal module connection.

    3.

  3. This step tests for the correct power supply conditions of the suspect module or modules.

    4.

  4. This step identifies which of the GMLAN busses the module or modules use for communication. Use the Data Link References for identifying the correct buss to analyze.

  5. The module which was not communicating due to an open in the GMLAN serial data circuits may have set Loss of 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

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Attempt to communicate with body control module (BCM).

Were you able to communicate with BCM?

Go to Step 4

Go to Step 3

3

  1. Turn the ignition OFF.
  2. Disconnect the harness connectors from the BCM.
  3. Test the following circuits of the BCM harness connector for the indicated condition. Refer to Circuit Testing and Wiring Repairs .
    4. • Switched and constant B+ supply circuits, for power
    • Ground circuits, for continuity
    • All communications circuits, for continuity

Did you find and correct the condition?

Go to Step 9

Go to Step 7

4

Attempt to communicate with all of the modules on the vehicle.

Were you able to communicate with all of the modules?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 5

5

Test the following circuits of the modules that do not communicate:

    • Switched B+ supply circuits
    • Constant B+ supply circuits
    • Ground circuits

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6

  1. Refer to Data Link References to identify the GMLAN buss or busses that the modules use to communicate and make a note.
  2. If more than one module is not communicating, use the data link connector (DLC) schematic to determine which module is closest to the DLC. Start the analysis at that module. Refer to Data Link Connector Schematics .
  3. Disconnect the harness connectors from the non-communicating modules.
  4. Test for an open circuit in the appropriate buss between the GMLAN terminals of the disconnected harness connector and the DLC. Use the note from item 1 for buss identification.

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7

Inspect for poor connections at the harness connector of the non-communicating module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

Important: Perform the module setup procedure, if required.

Replace the non-communicating module. Refer to Control Module References .

Did you complete the replacement?

Go to Step 9

--

9
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Retrieve DTCs from the module that 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 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 Loss of Communications DTC set.

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

Go to Diagnostic System Check - Vehicle

Go to Step 12

12

  1. Attempt to communicate with all the modules on the vehicle.
  2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Does DTC U0020 set in any module?

Go to Step 2

System OK