GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

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

DTC Descriptor

DTC C0550: ECU Malfunction

Circuit/System Description

At each power-up, the transfer case shift control module runs a self-test on the following functions:

    â€¢ EEPROM Checksum
    â€¢ ROM Checksum
    â€¢ RAM Checksum
    â€¢ RAM Malfunction

Conditions for Running the DTC

    â€¢ The ignition is ON.
    â€¢ System voltage is 9-18 volts.

Conditions for Setting the DTC

    â€¢ Critical operational parameters stored in EEPROM have failed Checksum test, indicating invalid data.
    â€¢ Masked ROM code has failed Checksum test, indicating that masked ROM data is not currently valid.
    â€¢ ROM code has failed Checksum test, indicating that ROM data is not currently valid.
    â€¢ One or more ROM locations have failed operational test, indicating that some portions of RAM are not functional.

Action Taken When the DTC Sets

    â€¢ The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.
    â€¢ The transfer case control switch does not display the current position LED while DTC C0550 is current.

Conditions for Clearing the DTC

    â€¢ Replace the transfer case shift control module.
    â€¢ Perform the Diagnostic System Check - Vehicle .

Reference Information

Schematic Reference

Transfer Case Control Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Transfer Case Description and Operation

Electrical Information Reference

    â€¢  Circuit Testing
    â€¢  Connector Repairs
    â€¢  Testing for Intermittent Conditions and Poor Connections
    â€¢  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.