Motor Vehicle Expert
OBD ECU memory fault guide

P0604 Code Meaning UK

P0604 means “Internal Control Module Random Access Memory Error”. In plain English, the engine computer has detected a problem with its internal RAM memory operation. This can be caused by weak battery voltage, unstable charging, poor earths, ECU power feed faults, water ingress, connector corrosion, failed programming, software corruption or internal ECU failure.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Mechanic-style ECU advice
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✓ ECU RAM fault explained ✓ UK repair cost guide ✓ Battery and voltage checks ✓ Heavy internal linking included
Quick answer Meaning Symptoms Causes Can you drive? Repair costs Diagnosis MOT impact Related guides FAQs
Quick answer

What does P0604 mean?

P0604 means the ECU or PCM has detected an internal random access memory error. RAM is working memory used by the control module while the vehicle is operating.

This sounds serious, but the ECU should not be replaced until the basics are checked. Low voltage, unstable charging, poor earths, ECU power feeds, fuses, relays, water ingress and connector corrosion can all trigger or contribute to control-module memory faults.

Most important first check

Battery condition, charging voltage, earth straps, ECU power feeds, fuses and connectors.

Main risk

Limp mode, cutting out, no-start symptoms, poor running or repeated ECU memory faults.

Best advice

Do not replace the ECU until voltage, wiring, fuse and connector checks are complete.

Code meaning

P0604 — Internal control module random access memory error

The ECU uses internal RAM while the engine is running to process sensor data, control actuators, manage fuelling, communicate with other modules and monitor emissions systems.

P0604 is stored when the ECU detects a problem with that internal memory operation. This may be caused by internal module failure, but external voltage and wiring problems must be ruled out first.

Symptoms

Common symptoms of a P0604 code

Symptoms depend on whether the issue is a one-off voltage upset, a wiring/power feed problem or a genuine ECU internal memory fault.

Engine management light

A steady or recurring engine warning light is common.

Limp mode

The ECU may reduce power if it cannot trust internal module operation.

Cutting out

Power supply, ground or internal module faults can cause intermittent stalling.

No-start symptoms

If the ECU cannot operate correctly, the car may crank but not start.

Multiple warning lights

Voltage or module memory faults can trigger warning lights across different systems.

Many unrelated codes

Bad power supply or module operation can create fault codes that seem unrelated.

Common causes

What causes P0604?

P0604 can be caused by an internal ECU fault, but voltage and power supply faults are common starting points.

Battery

Weak battery voltage

Low voltage during cranking can make modules behave incorrectly.

Charging

Alternator or charging fault

Unstable charging voltage, spikes or undercharging can affect ECU memory operation.

Earth

Poor earth strap

Bad grounds can cause strange ECU, sensor and communication faults.

Power feed

ECU power supply fault

Faulty fuses, relays or wiring can interrupt module operation.

Water

Water ingress or connector corrosion

Moisture can damage ECU connectors, fusebox terminals and wiring looms.

Module

Internal ECU RAM failure

If external checks are good and P0604 keeps returning, internal ECU failure becomes more likely.

Safe to drive?

Can you drive with P0604?

Be careful. Short driving may be possible if the car starts, runs normally and has no serious symptoms. But if the car cuts out, struggles to start, enters limp mode, loses throttle response or shows multiple warning lights, it should be diagnosed before regular driving.

✅ Lower risk: steady warning light only, normal starting and normal charging voltage.

⚠️ Medium risk: repeated P0604, low voltage codes, intermittent limp mode or rough running.

🚫 Higher risk: cutting out, no-start, unstable voltage, water-damaged ECU area or severe poor running.

Can You Drive With Engine Light? Car Won’t Start Diagnosis
Voltage warning

Check battery and charging system first

P0604 may look like an ECU failure, but bad voltage can create module memory problems. Battery condition, alternator output and earth straps should be checked before expensive ECU work.

If P0604 appears with P0562, start with the battery, alternator, earth straps and ECU power feeds.

Battery Warning Light P0562 Voltage Low
UK repair costs

Typical UK repair costs for P0604

Costs vary because P0604 may be caused by a simple voltage issue or a genuine ECU internal RAM fault.

Diagnostic scan and voltage checks

Typical range: £60–£150.

Battery replacement

Typical range: £90–£250+ depending on battery type.

Fuse, relay or power feed repair

Typical range: £50–£250+.

Earth strap or wiring repair

Typical range: £80–£300+.

ECU testing or repair

Typical range: £150–£600+.

ECU replacement and programming

Typical range: £500–£1,500+ depending on model, coding and immobiliser work.

Repair Costs Guide UK
Diagnosis flow

How to diagnose P0604 properly

✅ Scan all modules, not only the engine ECU.

✅ Record stored, pending, permanent and communication codes.

✅ Check whether P0604 appears with P0562, P0601, P0602, P0603, P0606 or P0607.

✅ Test battery voltage before start-up and during cranking.

✅ Check alternator charging voltage and voltage stability.

✅ Inspect battery terminals and main earth straps.

✅ Check ECU power feeds, fuses and relays.

✅ Inspect ECU connectors for corrosion, water ingress or loose pins.

✅ Check for water leaks near ECU, fusebox or wiring looms.

✅ Consider ECU testing only after external power and wiring checks pass.

Real experience

What I would check first

On a real P0604 fault, I would start with battery and charging checks. I would want to know whether the module has clean power during cranking and running.

If voltage and grounds are good, I would inspect ECU connectors for corrosion or water damage. A repeated P0604 after good power supply checks is when specialist ECU testing becomes more sensible.

P0603 Keep Alive Memory P0606 Processor Fault P0607 Performance Fault
MOT impact

Will P0604 fail an MOT?

P0604 itself is not usually the direct MOT failure. The concern is what the memory fault causes. If it causes an emissions-related engine warning light, poor running, cutting out, smoke, no-start symptoms or unsafe behaviour, it can become an MOT issue.

⚠️ Higher risk: engine warning light, no-start, cutting out, poor running or multiple warning lights.

⚠️ Medium risk: low-voltage faults, repeated ECU memory codes or intermittent limp mode.

✅ Lower risk: repaired voltage/module issue, no warning light and normal starting/running.

Engine Light MOT Guide Common MOT Failures
Used car buying advice

P0604 on a car you want to buy

If a used car has P0604 stored, be careful. Check for battery problems, warning lights, no-start history, water leaks, ECU repairs, module communication faults and whether the code returns after clearing.

A voltage-related P0604 may be manageable. A repeated internal ECU memory fault with no battery or wiring issue can become expensive.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0604

Common questions about P0604 internal control module random access memory error, safe driving, repair costs and MOT impact.

What does code P0604 mean?

P0604 means the ECU or PCM has detected an internal control module random access memory error.

Can I drive with P0604?

Short driving may be possible if the car runs normally, but no-start, limp mode, cutting out or multiple warning lights should be checked quickly.

Can a weak battery cause P0604?

Yes. Weak battery voltage, unstable charging voltage or poor earths can cause ECU memory and module performance faults.

Does P0604 mean the ECU is faulty?

Not always. Battery, charging, earth, fuse, relay, wiring and connector faults should be checked before ECU replacement.

Will P0604 fail an MOT?

It can contribute to an MOT issue if it causes an engine warning light, poor running, emissions problems, no-start symptoms or unsafe behaviour.

Should I replace the ECU first?

No. Check battery voltage, charging voltage, ECU power feeds, grounds, fuses, relays, connectors and water damage first.

About this guide

Written for practical UK fault finding

Motor Vehicle Expert explains diagnostic trouble codes in clear, mechanic-style language for UK drivers. This P0604 guide is designed to help you understand internal control module RAM errors, likely causes, safe driving advice, repair costs and used-car buying risks before replacing expensive control modules.

Fault codes should always be treated as a diagnostic starting point. P0604 can involve ECU memory, battery voltage, alternator output, earth straps, ECU power feed, fuses, relays, connectors, water ingress and internal module faults, so proper testing is better than guessing.