For 1990-2009 cars

### 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 Descriptors

DTC C0550 00: Electronic Control Unit (ECU) Performance
DTC C0550 31: Electronic Control Unit (ECU) Performance
DTC C0550 32: ECU Performance General Memory Failure
DTC C0550 34: ECU Malfunction RAM Failure
DTC C0550 35: ECU Malfunction ROM Failure
DTC C0550 36: ECU Malfunction EEPROM Failure
DTC C0550 37: ECU Malfunction Watch Dog failure
DTC C0550 38: ECU Performance Supervision Software Failure
DTC C0550 39: ECU Performance Internal Electronic Failure/Discard
DTC C0550 4B: ECU Internal Fault

### Circuit/System Description

When the ignition is turned ON, the microprocessor runs a diagnostic program to detect an internal fault when power up is commanded. If an internal malfunction is detected, the control module will store a DTC C0550 in its memory.

### Conditions for Running the DTC

The microprocessor runs a diagnostic program to detect an internal fault when power up is commanded. The only requirements are voltage and ground.

### Conditions for Setting the DTC

• | The control module detects an internal write malfunction.

• | The control module detects an internal checksum malfunction.

### Action Taken When the DTC Sets

• | The affected system warning indicator/warning message will activate.

• | The affected system will be disabled.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the control module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

### Circuit/System Verification

1. With a scan tool, clear DTC C0550. The DTC should clear.

⇒ | If the DTC will not clear, replace the control module.

2. Ignition OFF, remove the scan tool, open and close the drivers door.
3. Wait 60 seconds, ignition ON, then recheck for DTCs. DTC C0550 should not set.

⇒ | If DTC C0550 sets, replace the module.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for control module replacement, setup, and programming