What's the difference between P0604 and P0603?

Both are PCM-internal memory codes, but they involve different memory. P0603 is a Keep Alive Memory (KAM) error — the adaptive, learned data retained between key cycles, very often caused by a weak battery or bad ground. P0604 is a RAM error — the volatile working memory the PCM uses for live calculations while running. P0604 leans more toward genuine module hardware failure, though power and grounds should still be checked first.

Does P0604 mean I need a new PCM?

More often than the other internal-memory codes, yes — but not automatically. RAM faults frequently indicate a failing memory chip or processor interface inside the module. However, chronic low voltage, a bad ground, or a voltage spike can also trip a RAM error, and those are far cheaper to fix. Confirm the battery, terminals, and grounds are good and rule out any recent voltage event before authorizing a replacement.

Can I keep driving with P0604?

It depends on how the engine is behaving. If it runs normally and the only symptom is the check engine light, you can drive to arrange diagnosis — but don't rely on it for important trips, because RAM faults can escalate and leave you stranded. If the engine is in limp mode, running erratically, stalling, or hard to start, have it diagnosed promptly and consider towing rather than risking a no-start away from home.

Can a bad battery really trip a RAM error?

Yes. While P0604 points more toward hardware than the KAM code does, an unstable power supply can still destabilize the module enough to fail a memory self-check. Chronic low voltage, a marginal ground, or a rough jump-start can all trigger it. That's exactly why checking the battery, terminals, and grounds is the first step — it's the cheapest possible fix, and skipping it can lead to an unnecessary PCM replacement.

OBD-II trouble code

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

The PCM detected a fault in its internal RAM — the working memory it uses for live calculations while the engine runs. Unlike the learned-data codes, this points more strongly at the module hardware itself, though power and ground problems should be ruled out first.

Quick facts

System: Powertrain
Category: PCM / Electronics
Severity: High severity
Drivable: Usually safe to drive short-term
Repair cost range: $100 – $1,500
DIY difficulty: Shop recommended

What does P0604 mean?

RAM (Random Access Memory) is the PCM's working scratchpad — the volatile memory it uses to hold and manipulate live data while the engine is running: sensor readings, intermediate calculations, and the moment-to-moment values that drive fueling, timing, and other control decisions. P0604 sets when the module's self-diagnostics find that this RAM isn't reading or writing correctly. Because RAM is where the computer does its real-time thinking, a fault here means some of those live calculations can't be trusted.

P0604 leans more toward genuine internal hardware trouble than the adaptive-memory code P0603 does. RAM faults are often the result of the memory chip or the processor's memory interface failing, sometimes aggravated by heat, vibration, water intrusion, or age. That said, the power side still matters: chronic low voltage, a marginal ground, or voltage spikes from a rough jump-start can all destabilize the module enough to trip a RAM error, and those are far cheaper to fix than the computer. So while P0604 is more likely than the KAM code to end in a module replacement, the power supply and grounds should still be confirmed before condemning the PCM.

Depending on how severe the fault is, the engine may run normally with just the check engine light on, or it may drop into limp mode, run erratically, or fail to start. Intermittent RAM errors that come and go are common early in a module's decline. Treat P0604 as a code that warrants prompt professional diagnosis: if the power supply checks out and the fault persists or worsens, the PCM needs to be replaced and programmed.

Common causes

Internal PCM RAM or memory-interface hardware failure
Chronic low battery voltage destabilizing the module
Marginal or corroded PCM ground connection
Voltage spike or reverse-polarity jump-start damage
Water intrusion or corrosion at the PCM connector or housing
Heat or vibration damage to the module over time
Corrupted software in rare cases (reflash may help before replacement)

Symptoms

Check engine light on with P0604 stored
Engine may run normally or may drop into limp mode
Erratic running, hesitation, or stalling
Possible no-start or intermittent hard-start condition
Other PCM-internal or communication codes set at the same time
Symptoms may be intermittent at first and worsen over time

Diagnostic steps

1.Check battery voltage and the condition of the terminals and major grounds. Rule out the cheap power-side causes before suspecting the module.
2.Inspect the PCM connector and harness for corrosion, water intrusion, and pin damage.
3.Scan for accompanying codes; a group of PCM-internal codes (P0601-P0605) can suggest a shared power or grounding issue rather than a single failed chip.
4.Look for any recent voltage event — jump-start, dead battery, alternator problem — that could have damaged the module.
5.Check for a manufacturer technical service bulletin or reflash addressing P0604 on the vehicle; a software fix is occasionally available.
6.Clear the code and monitor whether it returns with confirmed-good power and grounds.
7.If P0604 persists with a healthy power supply, the PCM has an internal RAM failure and needs replacement and programming.

Repair cost

$100 – $1,500

If the cause turns out to be power-side — battery, terminals, or ground — the fix is $100-$300. A reflash, where applicable, is $80-$250. But because P0604 points more strongly at internal hardware, many cases end in PCM replacement plus programming at roughly $800-$1,500 (module $400-$1,200 plus programming and core charge). Confirm the power supply first, but be prepared for a module replacement if the fault persists.

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DIY vs shop

Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.