Low or unstable supply voltage to the control module, from a weak battery, poor charging or voltage drops; Poor module power or earth connections, corroded or loose connectors or terminals; Internal failure of the control module memory or processor; Water ingress or corrosion inside the module or its connector; Outdated or corrupted module software, where a known fault has been addressed by a manufacturer update; Electrical interference or a short in a circuit supplied by the module, such as a faulty sensor or actuator pulling the supply down; Interrupted software update or previous incorrect programming.

P0604

Internal Control Module Random Access Memory (RAM) Error.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
04: FaultThe specific fault in that area

What it means

P0604 means the control module (usually the engine control unit, though the SAE J2012 definition covers any powertrain module) found an error during its internal self-test of its Random Access Memory (RAM). The module reads and writes test values to its memory and sets the code when the results do not match what was expected. It can be a genuine internal fault or a temporary glitch caused by supply voltage problems.

Symptoms

Engine warning light on
Engine cranks but fails to start, or starts and stalls
Rough running, misfiring or hesitation
Car goes into limp mode with reduced power
Gearbox behaving erratically if the transmission control module is the one reporting
Intermittent fault that clears and returns

Possible causes

Low or unstable supply voltage to the control module, from a weak battery, poor charging or voltage dropslikely
Poor module power or earth connections, corroded or loose connectors or terminalslikely
Internal failure of the control module memory or processorlikely
Water ingress or corrosion inside the module or its connectorpossible
Outdated or corrupted module software, where a known fault has been addressed by a manufacturer updatepossible
Electrical interference or a short in a circuit supplied by the module, such as a faulty sensor or actuator pulling the supply downrare
Interrupted software update or previous incorrect programmingrare

How to diagnose

Read all stored and pending codes from every module and note the freeze frame data, looking for other codes that point to voltage or communication problems
Check the battery state of charge and condition, then test charging system output with the engine running
Inspect the module connectors and housing for corrosion, moisture, bent pins and damage
Check the module power supply and earth circuits for voltage drop under load, following the wiring diagram for that model
Check for loose or corroded battery terminals and main earth straps
Clear the code and road test to see if it returns straight away or only intermittently
Check for available software updates or technical bulletins for the specific vehicle and engine
If everything else checks out and the code returns immediately, confirm the module as faulty before replacing it

How to fix

Replace or recharge a weak battery and repair any charging system fault
Repair or clean wiring, connectors, terminals and earth points that show voltage drop or corrosion
Dry out and repair the cause of any water ingress, then replace the module if it has been damaged
Update the module software where a manufacturer update addresses the fault
Repair or replace the module, with correct coding and programming to the vehicle as the model requires

Can I keep driving?

Medium severity

The car may run normally or may drop into limp mode or stall without warning, so avoid long journeys and book it in soon. Stop driving if it stalls, will not hold idle or loses power, as that can be unsafe in traffic.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.