GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC U2100: Controller Area Network (CAN) Bus Communication

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

CAN High

U2104

U2106/U2108

U2104

--

CAN Low

U2104

U2106/U2108

U2104

--

Circuit/System Description

Modules connected to the CAN serial data circuits monitor communications. During normal vehicle operation, information and commands are exchanged among modules. If the ECM fails to receive an expected message from one of the other modules, DTC U1800, U2106, or U2108 will set; depending on what communication is lost. If no communication is detected on the CAN serial data bus, a DTC U2100 is set. A no communication condition may result from a short to voltage, short to ground, or short between the CAN high and low circuits.

Conditions for Running the DTC

    • The ignition 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) illuminates.

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

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.

Reference Information

Schematic Reference

Data Communication Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool References

Control Module References for Scan Tool Information

Circuit/System Testing

  1. Ignition OFF, disconnect all module harness connectors and external resistor connectors containing the CAN high and CAN low circuits.
  2. Ignition ON, verify that a test lamp does not illuminate between the CAN high serial data circuit terminals and ground, and the CAN low serial data circuit terminals and ground of all connectors.
  3. If the test lamp illuminates, test the CAN serial data circuit for a short to voltage.
  4. Ignition ON, verify that a test lamp does not illuminate between the CAN high serial data circuit terminals and B+, and the CAN low serial data circuit terminals and B+ of all connectors.
  5. If the test lamp illuminates, test the CAN serial data circuit for a short to ground.
  6. Test for infinity between the CAN high serial data circuit terminals and the CAN low serial data circuit terminals at all harness connectors.
  7. If less than the specified value, test for a short between the CAN high and low serial data circuits.
  8. Ignition OFF, remove the CAN high serial data circuit and CAN low serial data circuit terminals listed from the harness connector X1 of the ECM. Reconnect the harness connector X1 of the ECM.
  9. RPO LG4

    • Terminal 17
    • Terminal 18

    RPO LG5

    • Terminal 50
    • Terminal 34

    Important: After each frequency test, disconnect the connector of the module tested to isolate the next module to be tested. Ensure the ignition is OFF prior to any module connect or disconnect.

  10. Reconnect each module, one at a time, and with the engine running measuring for greater than 35,000 Hz (frequency) between the CAN serial data circuit terminals that were removed in the previous step.
  11. If less than the specified range, replace the module that caused the low reading.
  12. If all circuits test normal, replace ECM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup and programming