GM Service Manual Online
For 1990-2009 cars only

Circuit Description

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

    • EEPROM Checksum
    • ROM Checksum
    • RAM Checksum
    • RAM Malfunction
    • Relay Malfunction

Conditions for Setting the DTC

    • Critical operational parameters stored in EEPROM have failed Checksum test indicating invalid data.
    • Masked ROM code has failed Checksum test indicating that masked ROM data is not valid.
    • ROM code has failed Checksum test indicating that ROM data is not valid.
    • One or more ROM locations have failed operational test indicating that some portions of RAM are not functional.
    • Motor circuit test has indicated that one of the modules's relays is not able to make contact.

Action Taken When the DTC Sets

The SERVICE indicator (4WD/AWD) lamp will be latched on and the system will be disabled.

Conditions for Clearing the DTC

    • Replace the transfer case shift control module.
    • Perform the Diagnostic System Check- Transfer Case.

Test Description

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

  1. Replacement of the transfer case shift control module.

DTC C0550 ECU Malfunction

Step

Action

Yes

No

Schematic Reference: Transfer Case Control Schematics

1

Did you perform the Transfer Case Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Transfer Case

2

Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement .

Is the repair complete?

System OK

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