P0604: Internal Control Module Random Access Memory (RAM) Error

P0604 means the module's working memory (RAM) failed its self-test: a value written to a cell does not read back correctly, a built-in hardware test fails, error correction reports faults, or protected memory is written without permission. GM's engine module scans the whole RAM within 30 s of start-up and then keeps checking in the background, setting the code after repeated failures with the lamp in the first trip; Ford tests RAM during initialisation, the GM gateway with error correction at start (two trips). Unlike most memory codes, P0604 is used the same way by GM, Ford, Chrysler and Saab. Consequences depend on the module: a transmission module goes into limp mode with maximum line pressure and a fixed gear, an engine module may reduce power. The code alone does not say which module set it – the scan tool shows whether it is the engine, transmission, brake or body module.

Symptoms

• Check engine light on
• Limp mode or transmission limp mode
• Intermittent stalling or starting problems
• Several module codes at the same time

Most likely causes

• Internal hardware defect of the module (RAM)
• Low voltage, voltage dips or a poor ground or supply at the module
• Faulty module software (update available)
• Moisture or corrosion in the module or at its connector

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the battery and grounds
2. Clear the code and see if it returns
3. If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
4. Check whether a software update is available
5. Do not replace the control module without a full diagnosis

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P0604 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 6: Control modules and outputs
• 04: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0604?

It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

What causes code P0604?

The most common causes are: internal hardware defect of the module (RAM); low voltage, voltage dips or a poor ground or supply at the module; faulty module software (update available).

Related codes

• P0601Internal Control Module Memory Checksum Error
• P0603Internal Control Module Keep Alive Memory (KAM) Error
• P0605Internal Control Module Read Only Memory (ROM) Error
• P0606Control Module Processor
• P0607Control Module Performance
• P062FInternal Control Module EEPROM Error

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 25OBDG03A
• newtonnet.co.uk: fordobd2012
• saabwisonline.com: dtc p0601 p0602 p0603 or p0604

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.