Motor Vehicle Expert
OBD ECU memory fault guide

P0603 Code Meaning UK

P0603 means “Internal Control Module Keep Alive Memory Error”. In plain English, the engine computer has detected a problem with the memory that stores learned settings and important data when the ignition is switched off. This can be caused by weak battery voltage, battery disconnection, poor earths, blown fuses, ECU power supply faults, wiring damage, water ingress, software issues or internal ECU memory failure.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Mechanic-style ECU advice
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✓ ECU memory 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 P0603 mean?

P0603 means the ECU or PCM has detected a problem with its keep alive memory. Keep alive memory stores learned values, adaptations and important data even when the ignition is off.

This code often appears after battery disconnection, weak battery voltage, poor charging voltage, blown fuses, ECU power supply interruptions or module memory problems. It does not automatically mean the ECU needs replacing.

Most important first check

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

Main risk

Lost learned settings, poor idle, no-start symptoms, warning lights or repeated module faults.

Best advice

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

Code meaning

P0603 — Internal control module keep alive memory error

The ECU keeps some memory alive even when the ignition is off. This can include fuel trims, idle learning, adaptation values, readiness data and other learned settings.

P0603 is stored when the ECU detects that this memory has been lost, corrupted or cannot be maintained correctly. A simple battery or power supply fault can cause it, but repeated P0603 faults need deeper electrical and module checks.

Symptoms

Common symptoms of a P0603 code

Symptoms depend on whether the fault is a one-off voltage memory loss or a repeated ECU/module fault.

Engine management light

A steady engine warning light is common when P0603 is stored.

Poor idle

Lost idle learning or adaptations can cause unstable idle after battery or ECU memory loss.

Hard starting

Some cars may start poorly while learned values are missing or relearning.

Multiple warning lights

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

Failed readiness monitors

Emissions readiness data may reset after memory loss.

Code returns after clearing

If P0603 keeps returning, check constant power feed, grounds and ECU condition.

Common causes

What causes P0603?

P0603 is often linked to memory power supply, battery voltage or ECU keep-alive data.

Battery

Weak or disconnected battery

Battery disconnection or low voltage can clear learned ECU memory and trigger P0603.

Voltage

Charging system fault

Unstable alternator output, voltage drops or spikes can affect module memory.

Fuse

Blown fuse or poor ECU power feed

The ECU may lose its constant memory supply if a fuse, relay or power feed is faulty.

Earth

Poor earth strap

Bad grounds can create strange control module and memory-related faults.

Water

Water ingress or connector corrosion

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

Module

Internal ECU memory fault

If power supply and wiring are good, the ECU memory circuit itself may be failing.

Safe to drive?

Can you drive with P0603?

Short careful driving may be possible if the car starts, runs normally and the code appeared after a battery change or flat battery. But if the car is cutting out, struggling to start, showing multiple warning lights, entering limp mode or repeatedly storing P0603, it should be checked properly.

✅ Lower risk: code appeared after battery disconnection and the car runs normally.

⚠️ Medium risk: rough idle, poor starting, repeated code or low voltage code stored.

🚫 Higher risk: cutting out, no-start, limp mode, unstable voltage or water-damaged ECU area.

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

Battery and voltage faults must be checked first

P0603 is one of those codes where the battery and charging system matter a lot. A weak battery, poor earth or faulty alternator can make the ECU lose memory even when the ECU is not internally faulty.

If P0603 appears with P0562, start with battery voltage, charging voltage, earth straps and ECU power feeds.

Battery Warning Light P0562 Voltage Low
UK repair costs

Typical UK repair costs for P0603

Costs vary because P0603 may be a simple battery memory loss issue or a genuine ECU memory 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 P0603 properly

✅ Scan all modules, not only the engine ECU.

✅ Record stored, pending, permanent and communication codes.

✅ Ask whether the battery was recently disconnected, flat or replaced.

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

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

✅ Check alternator charging voltage and voltage stability.

✅ Check ECU constant power feed and memory power supply.

✅ Inspect fuses, relays, earth straps and main grounds.

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

✅ Confirm whether P0603 returns after proper voltage and power-feed checks.

Real experience

What I would check first

On a real P0603 fault, I would first ask whether the battery went flat or was disconnected. If the code appeared straight after that and the car now runs normally, it may simply need clearing and a drive cycle.

If the code returns, I would check constant ECU power, grounds, fuses and connector condition. A memory fault that keeps returning after good voltage checks is when ECU testing becomes more serious.

P0601 Memory Fault P0602 Programming Error P0607 Performance Fault
MOT impact

Will P0603 fail an MOT?

P0603 itself is not usually the direct MOT failure. The concern is what the memory or voltage 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, poor running, limp mode or multiple warning lights.

⚠️ Medium risk: recent battery failure, memory reset, voltage faults or repeated ECU memory codes.

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

Engine Light MOT Guide Common MOT Failures
Used car buying advice

P0603 on a car you want to buy

If a used car has P0603 stored, ask whether the battery was recently replaced or disconnected. Also check for poor starting, warning lights, water leaks, ECU repairs, voltage codes and whether the fault returns after clearing.

A one-off memory code after a flat battery may be simple. A repeated P0603 with no voltage issue can become expensive if ECU testing or replacement is needed.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0603

Common questions about P0603 internal control module keep alive memory error, safe driving, repair costs and MOT impact.

What does code P0603 mean?

P0603 means the ECU or PCM has detected an internal control module keep alive memory error.

Can I drive with P0603?

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 P0603?

Yes. Weak battery voltage, battery disconnection or unstable charging voltage can cause keep alive memory faults.

Does P0603 mean the ECU is faulty?

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

Will P0603 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 constant power feed, 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 P0603 guide is designed to help you understand internal control module keep alive memory 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. P0603 can involve ECU memory, battery voltage, alternator output, earth straps, constant power feed, fuses, relays, connectors, water ingress and internal module faults, so proper testing is better than guessing.