P0603
PowertrainInternal Control Module Keep Alive Memory (KAM) Error
Most of the time this is a power supply problem rather than a dead ECU, usually a tired battery or grubby terminals that can't hold steady voltage to the module. The ECU keeps a chunk of memory alive permanently, even with the key off, where it stores everything it has learned about your fuelling, timing and gearbox shifts. When that memory fails its self-test on startup, often because power dropped out while it was writing, P0603 gets logged. So before anyone starts talking about replacing the ECU, the battery and its connections need ruling out first.
ⓘ Information only. This page provides general educational information about fault code P0603. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0603 mean?
P0603 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Internal Control Module Keep Alive Memory (KAM) Error.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on the dash, sometimes with no other obvious change to how the car drives
- • Long cranking before it catches, especially first thing in the morning
- • Lumpy idle on a cold start that smooths out once it warms through
- • Flat spot or hesitation when you put your foot down
- • Gearbox shifting harshly or hunting between gears, since the adaptive shift data has been wiped
- • Fuel economy that drops off until the ECU relearns its trims
Possible causes
Causes commonly associated with P0603, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Weak or failing battery that can't hold the constant low-level voltage the memory needs, by far the most common trigger and the first thing to check
- 2. Corroded or loose battery terminals interrupting the supply, easy to miss and easy to fix
- 3. A blown fuse on the ECU's permanent live feed, often caused by a dodgy accessory wired into the cigarette lighter circuit
- 4. Chafed, frayed or corroded wiring on the way to the ECU, common on older cars where looms have been disturbed during other repairs
- 5. A poor earth connection at the module, which causes the same symptoms as a power supply fault
- 6. An aftermarket remap or plug-in tuning box upsetting the stored settings
- 7. A faulty PCM or ECU, the least likely cause and the one you only land on after the cheap stuff checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Load test the battery and check it properly. You want over 12.4V rested with the engine off and a steady 13.8 to 14.4V running. A battery that reads fine at rest but collapses under load is a classic cause of this code
- 2. Pull the terminals off, clean the posts and clamps back to bright metal, then refit them tight. Five minutes of work that clears a fair share of these
- 3. Read the freeze frame data alongside the code so you can see what the engine was doing when it logged, and note any companion codes
- 4. Check the fuses feeding the ECU's permanent live, particularly the always-on memory feed, and look for anything aftermarket spliced into nearby circuits
- 5. Inspect the loom and connectors running to the module for damage and corrosion, and confirm the earth strap is clean and torqued down. A multimeter voltage-drop test on the earth tells you quickly if it's the problem
- 6. If a tuning box or remap is fitted, return it to standard and see whether the code stays gone before you condemn the ECU itself
Common questions about P0603
What actually goes wrong if I just keep driving with this on? +
If the cause is a flat battery or a loose terminal, you'll mostly notice annoyances like slow starts, a rough cold idle and slightly worse mpg while the ECU keeps relearning everything it forgot. Where it gets dicey is if the module is dropping its memory at random while you're moving, because that can throw the car into limp mode or stall it. So it's not the kind of fault that strands most people, but an intermittent stall in traffic is exactly what you don't want. Sort the battery first and a lot of the worry goes away.
How quickly do I need to deal with it? +
Treat it as a within-a-week job rather than a roadside emergency, assuming the car is starting and driving normally otherwise. If you're getting stalling, hard starting or the gearbox is behaving badly, bump it up the list because those point to the memory dropping out under power rather than just an old battery. The longer you ignore a weak battery feeding the ECU, the more often the memory gets corrupted and the more relearning the car has to keep doing.
Is it the ECU itself that's gone, or just the wiring and battery feeding it? +
On the vast majority of P0603s it is the supply side, meaning the battery, the terminals, an earth or a fuse, not the ECU's internal circuitry. The module needs steady, uninterrupted voltage to hold its memory, and anything that lets that voltage drop will trip this code even though the ECU is perfectly healthy. A failed control module does happen, but you should only spend money on one after the battery, earths and looms have all been proven good. Replacing an ECU on a hunch is an expensive way to find out the battery was on its way out.
How long does the repair usually take? +
A battery, terminal clean or fuse fix is half an hour or less and you're driving away the same day. Tracing a wiring or earth fault is harder to pin a time on, anything from an hour to half a day depending on how buried the damaged section is. If it comes down to replacing and programming the ECU, that's a longer job because the new unit often has to be coded to the car and matched to the keys, which usually means leaving it with a garage for the day.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →