P0604
PowertrainInternal Control Module Random Access Memory (RAM) Error
The engine control module has its own internal RAM, the short-term memory it uses to hold and process all the live data while the engine runs. P0604 means the module has failed a self-check on that memory. Because the ECU runs everything from fuelling to ignition timing to gear shifts, a genuine RAM fault can make the car stall, drop into limp mode, or refuse to start. Before you panic about a dead ECU though, this code is set far more often by dirty power supply than by an actual broken module.
ⓘ Information only. This page provides general educational information about fault code P0604. 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 P0604 mean?
P0604 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Internal Control Module Random Access Memory (RAM) 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 obvious change to how the car drives
- • Stalling or hesitation, usually worst at idle or just as you pull away
- • Car drops into limp mode with the power capped and the revs held down
- • Harsh, confused gear changes on automatics, or the box stuck in one gear
- • Long cranking or a flat-out no-start where the ECU won't wake up properly
- • Throttle feels woolly and slow to respond
Possible causes
Causes commonly associated with P0604, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. A tired or low battery is the number one trigger. When voltage sags, the ECU's memory check fails even though the module itself is fine
- 2. Corroded or loose battery terminals, same effect as a weak battery because the voltage at the ECU goes all over the place
- 3. Dodgy charging system. A failing alternator that puts out unstable or spiking voltage can upset the module mid-drive
- 4. Poor earth connections or chafed wiring at the ECU letting voltage drop where it shouldn't
- 5. Software corruption inside the module, often after a botched update or a battery going flat mid-flash
- 6. A blown or corroded fuse in the ECU power feed
- 7. An actual failed control module that needs replacing. This is the least common cause and should be the last thing you blame
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. Check the battery before you touch anything else. Resting voltage should be over 12.4V, and it should hold above 9.5V while cranking. A weak battery is the usual culprit and the cheapest to rule out
- 2. Pull every stored code and note the order they logged in. P0604 alongside other module codes points at a power or earth problem rather than a single dead part
- 3. Inspect the battery terminals and the main engine earth straps for corrosion, green fur, or a loose clamp
- 4. Run the engine and measure charging voltage. You want a steady 13.5 to 14.5V. Anything jumping around or below that means the alternator is suspect
- 5. Clear the code and road test. If it stays away, you were chasing a voltage glitch, often from a jump-start or a battery that went flat
- 6. If it comes straight back with healthy power and earths, check the ECU connectors and loom for damage or water ingress before condemning the module
Common questions about P0604
Can I keep driving the car like this? +
I wouldn't make a habit of it. With a live P0604 the car can stall without warning or drop into limp mode while you're moving, which is no fun on a busy roundabout or pulling out of a junction. Some cars won't restart once they've shut down. If it's just thrown the light but drives normally, get the battery checked sharpish and don't take it on a long motorway run until you know what set it. If it's stalling or going into limp mode, get it recovered rather than risk it in traffic.
Is this going to fail my MOT? +
The code on its own isn't tested, but the engine warning light is. If the MIL is lit when the tester plugs in or eyeballs the dash, that's an automatic emissions failure on most cars from 2018 onwards. Sort the underlying fault first, clear the code, then drive a few cycles so you're sure the light stays off before you book it in. Turning the light off five minutes before the test doesn't work, the tester checks the lamp behaviour at key-on too.
What's this likely to cost me? +
Depends entirely on what's actually behind it. A loose terminal or a blown fuse is a £20 to £40 job, often just labour at an independent. A new battery runs roughly £80 to £150 fitted, an alternator £200 to £400 at a good independent garage. If the module itself is dead and needs supplying, coding and reprogramming, you're into £500 to well over £1,000, and a main dealer will sit at the top of that. Most P0604s never get near that, they're fixed at the battery, so insist on the cheap checks first.
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 →