P0603 Control module P0604 Control module P0605 Control module

The P0603, P0604, and P0605 trouble codes indicate different types of internal memory errors within the Powertrain Control Module (PCM), also known as the Engine Control Module (ECM). Since all three codes point to internal memory faults, they strongly suggest that the PCM itself is defective and needs to be replaced or reprogrammed. 

Code definitions

• P0603—Internal Control Module Keep Alive Memory (KAM) Error: The PCM's KAM stores learned vehicle settings for optimal performance based on your driving habits. This code sets when the KAM fails its self-test, often caused by a power interruption.
• P0604—Internal Control Module Random Access Memory (RAM) Error: The PCM's RAM is its temporary memory, storing information needed for day-to-day operation. This code is logged if the PCM detects a problem with the RAM portion of its memory during a self-check.
• P0605—Internal Control Module Read Only Memory (ROM) Error: The PCM's ROM contains the core software and factory settings that cannot be modified. A P0605 indicates a serious internal problem with the permanent memory, which can corrupt the PCM's operating system. 

Common causes

When all three of these memory codes appear simultaneously, it's highly indicative of a critical failure within the PCM itself.

• Failed PCM: A single failure or corruption within the PCM's internal hardware can trigger multiple memory-related codes.
• Power or ground issues: Electrical problems, such as a weak battery, corroded battery terminals, or a faulty PCM power supply circuit, can corrupt the PCM's memory.
• Water damage: Moisture entering the PCM or its connectors can cause internal corrosion and damage to the circuit board.
• Aftermarket modifications: Installing aftermarket tuning chips can interfere with the PCM's programming, causing a memory malfunction. 

Common symptoms

If you are experiencing these codes, you may notice one or more of the following symptoms:

• An illuminated Check Engine Light on the dashboard.
• The vehicle may fail to start or experience rough idling and stalling.
• Poor or erratic performance, including issues with the engine's timing, fuel delivery, or transmission shifting.
• In some cases, the vehicle may enter "limp mode," which limits speed and engine power to prevent further damage.
• Other warning lights, such as the Anti-lock Brake System (ABS) or traction control lights, may also be illuminated. 

How to diagnose and repair

Attempting to diagnose and repair internal PCM faults is often beyond the scope of a typical home mechanic and may require professional assistance.

1. Check the battery and grounds: Begin by performing a thorough inspection of the battery, including its voltage and the condition of its cables and terminals. Also, check all the ground connections for the PCM and engine.
2. Inspect for water damage: Carefully examine the PCM's case and wire harnesses for any signs of water intrusion or corrosion.
3. Perform a reset: If no external issues are found, clear the codes with an OBD-II scanner and test-drive the vehicle to see if they return. If the codes come back immediately, the problem is likely internal to the module.
4. Reprogram or replace the PCM: If the codes reappear after checking for other issues, the PCM is likely faulty. A professional can attempt to reprogram or "reflash" the module. However, if the internal hardware is damaged, it will need to be replaced with a new or refurbished unit.