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. 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 U2104 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.

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

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.

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

Scan Tool References

Control Module References for Scan Tool Information

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.

⇒	If the test lamp illuminates, test the CAN serial data circuit for a short to voltage.

3. 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.

⇒	If the test lamp illuminates, test the CAN serial data circuit for a short to ground.

4. Test for infinity between the CAN high serial data circuit terminals and the CAN low serial data circuit terminals at all harness connectors.

⇒	If less than the specified value, test for a short between the CAN high and low serial data circuits.

5. Ignition OFF, remove the CAN high serial data circuit terminal 17 and the CAN low serial data circuit terminal 18 from the harness connector X1 of the ECM. Reconnect the harness connector X1 of the ECM.

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.

6. 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.

⇒	If less than the specified range, replace the module that caused the low reading.

7. If all circuits test normal, replace ECM.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup and programming