P0600 to P0605: control module memory and internal faults

Most codes describe something the module observed. This range describes something the module found wrong with itself, which is a different kind of claim and deserves a sceptical reading.

Code	Meaning
P0600	Serial communication link malfunction
P0601	Internal control module memory checksum error
P0602	Control module programming error
P0603	Internal control module KAM error
P0604	Internal control module RAM error
P0605	Internal control module ROM error

Related: P0606 PCM processor fault.

Do not buy a module yet

Control modules are expensive, often need programming to the vehicle, and are sometimes VIN-locked. They are also, in the great majority of these cases, not the fault.

Three things make a perfectly healthy module report internal errors.

1. Low or unstable supply voltage

A module running on a sagging supply produces exactly these symptoms. Memory operations fail, checksums do not match, and the module logs an internal error truthfully — the error happened, but the cause was outside.

Check system voltage under load before anything else: P0560 to P0563 system voltage and alternator charging diagnostics.

2. Poor grounds and connections

Corroded ground straps and connector pins do the same thing more subtly. A module with a marginal ground has an unstable reference for everything it does.

3. Something that happened during a battery event

Jump starting with reversed polarity, a battery disconnected while the engine was running, or welding on the vehicle without disconnecting the battery. All of these can genuinely damage a module, and all of them are worth asking about.

How to jump start a car correctly.

P0603: the keep-alive memory code

Worth separating out, because it is often benign.

Keep-alive memory holds learned adaptations — fuel trims, idle position, transmission shift adaptations — through the ignition being off. It is powered by a permanent battery feed.

P0603 frequently means nothing more than that the battery was disconnected. Learned values were lost, the module noticed, and it logged the fact.

If it appeared right after a battery change, clear it and drive. If it returns without a battery event, the permanent feed is failing — a fuse, a corroded connector or a broken wire.

That permanent feed is worth checking because losing adaptations repeatedly causes real symptoms: rough idle after every start, odd transmission shifts, and fuel trims that never settle.

P0602: programming error

This one is genuinely specific. It means the module’s software was not written correctly or is incomplete — most often after a failed flash.

Causes:

• Voltage dropping during programming, which is why proper flashing uses a stabilised power supply.
• An interrupted session.
• Wrong software for the vehicle.

If this appeared after an ECU remap or a dealer software update, that is the cause: coding, programming and flashing explained and ECU remap detection.

The fix is re-flashing with correct software, not replacement.

What you may notice

• The engine will not start, or starts and runs poorly.
• Limp mode.
• Multiple unrelated codes across systems.
• Nothing at all, with the code appearing on a routine scan.

That last case is common with P0603 in particular.

The order of work

1. Record every stored code before clearing anything.
2. Check system voltage at rest, cranking, and under load.
3. Inspect battery terminals and ground straps.
4. Ask what happened recently — battery change, jump start, remap, bodywork.
5. Clear the code and see whether it returns. A single occurrence after a battery event is usually nothing.
6. Check the permanent feed and its fuse if P0603 keeps returning.
7. Consider re-flashing before considering replacement.
8. Replace the module last, and only when supply and connections are proven good.

The pattern that changes the conclusion

If P0601 or P0604 appears repeatedly, is accompanied by genuinely erratic behaviour, and system voltage is verified stable across the whole drive, then the module is a real candidate.

That is a much narrower situation than “the scan tool said P0601”, and it is worth being precise about before spending.

When the scanner lies about ghost codes.

Pair an ELM327 adapter, open the free tier of the app and log control module voltage through a cold start. A supply that dips during cranking explains internal module errors far more often than a failed module does.