For 1990-2009 cars

### Circuit Description

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

• | EEPROM Checksum

• | ROM Checksum

• | RAM Checksum

• | RAM Malfunction

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC C0550 ECU Malfunction

### Conditions for Running the DTC

• | The ignition is ON.

• | The system voltage is 9-16 volts.

### 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 currently valid.

• | ROM code has failed Checksum test indicating that ROM data is not currently valid.

• | One or more RAM locations have failed operational test indicating that some portions of RAM are not functional.

### Action Taken When the DTC Sets

• | The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.

• | Transfer case shifting is disabled.

### Conditions for Clearing the DTC

• | Replace the transfer case shift control module.

• | Perform the Diagnostic System Check - Vehicle .

### Diagnostic Aids

Extremely low system voltage can cause this DTC to set falsely. Refer to Battery Charging .

### Test Description

The number below refers to the step number on the diagnostic table

1. This step replaces the transfer case shift control module.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Transfer Case Control Schematics Connector End View Reference: Transfer Case Control Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Replace the transfer case shift control module. Refer to Control Module References for replacement, setup, and programming. Did you complete the repair? | System OK | --