Motor Vehicle Expert
OBD control module memory fault guide

P0605 Code Meaning UK

P0605 means “Internal Control Module Read Only Memory Error”. In plain English, the engine control module, PCM or ECU has detected a problem with its internal memory check. It can point to a real ECU fault, but low voltage, poor earths, water damage, wiring problems, software corruption or previous jump-start damage should be checked before replacing the control module.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 ECU/PCM memory fault advice
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✓ ECU memory fault explained ✓ UK repair cost guide ✓ Voltage checks included ✓ Links back to indexed fault-code hub
Quick answer Meaning Symptoms Causes Can you drive? Repair costs Diagnosis Common mistakes MOT impact Used car advice Related guides FAQs
Quick answer

What does P0605 mean?

P0605 means the ECU, PCM or engine control module has detected an internal read-only memory error. The control module carries out self-checks, and when the stored memory data does not match what the module expects, it can set P0605.

This sounds serious because it involves the control module itself. However, the ECU should not be condemned straight away. Low battery voltage, charging problems, poor earth connections, water ingress, wiring faults, damaged connectors, failed programming or software corruption can all lead to ECU memory-related codes.

A proper diagnosis should prove whether the ECU is genuinely faulty or whether it is reacting to a poor electrical supply or external fault. This is why P0605 should be checked carefully before expensive ECU replacement or coding work.

Most important first check

Battery voltage, cranking voltage, alternator output, ECU earths, power feeds and connector condition.

Main risk

The car may enter limp mode, store multiple unrelated codes, cut out or refuse to start.

Best next step

Scan all modules and check voltage/earths before buying an ECU.

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Code meaning

P0605 — Internal Control Module ROM Error

The engine control module is effectively the computer that manages engine operation. It reads sensor information, controls injectors, ignition, throttle operation, emissions systems, idle control, fuel mixture, boost control and communication with other modules.

Inside the ECU is stored memory and software. ROM means read-only memory. When P0605 appears, the control module believes part of its stored internal memory, programming or self-check has failed.

On some cars, this can indicate a failing ECU. On others, it may appear after low voltage, a battery change, jump starting, poor charging voltage, water damage, coding issues or interrupted programming.

Mechanic view

Why P0605 needs careful fault finding

P0605 is not the kind of code where you should simply clear the light and ignore it. If the code returns, the control module has seen something it does not like internally.

At the same time, replacing an ECU without testing can be an expensive mistake. Many ECUs need programming, immobiliser matching, key coding and vehicle-specific configuration. If the real problem is low voltage, water ingress or a bad earth, a replacement ECU may still not fix the fault.

This is why P0605 links closely with P0601 internal control module memory checksum error, P0602 control module programming error, P0603 keep alive memory error, P0604 RAM error, P0606 processor fault and the main OBD fault codes hub.

Symptoms

Common symptoms of a P0605 code

Symptoms depend on whether the problem is a real ECU memory fault, software issue, voltage fault, wiring issue or communication problem.

Engine management light

A steady engine warning light is common, especially after the code returns following clearing.

Limp mode

The vehicle may reduce power if the ECU cannot trust its own internal operation.

No-start condition

If the ECU cannot initialise properly, the car may crank but fail to start.

Cutting out

Intermittent ECU power, memory faults or internal module issues can cause stalling.

Poor throttle response

Some cars may feel slow, restricted or unresponsive if the ECU enters a protective mode.

Multiple warning lights

ABS, traction, gearbox or immobiliser warnings may appear if module communication is affected.

Multiple unrelated codes

Low voltage or ECU faults can cause sensor and communication codes across different systems.

Intermittent starting

The car may start sometimes and fail at other times if the module fault is intermittent.

Failed emissions readiness

If ECU memory is unstable, readiness monitors may reset or fail to complete correctly.

Common causes

What causes P0605?

P0605 may be caused by internal ECU memory failure, but external electrical faults must be checked first.

Voltage

Low battery voltage

A weak battery can cause ECU self-check errors, especially during cranking or after long standing periods.

Charging

Alternator charging fault

Overcharging or undercharging can upset ECU operation and may damage sensitive electronics.

Earth

Poor earth strap

Bad grounds can cause unstable voltage, strange warning lights and control module fault codes.

Water

Water ingress

Moisture in ECU connectors, fuse boxes or wiring can create corrosion and memory-related ECU faults.

Wiring

ECU power feed problem

A bad relay, fuse, connector pin or damaged wire can interrupt the module power supply.

Software

Software or programming error

Failed updates, interrupted coding or incorrect programming can trigger control module memory errors.

Jump start

Voltage spike damage

Incorrect jump starting, reverse polarity or voltage spikes can damage ECU memory or circuitry.

Connector

Corroded ECU connector

Green corrosion, loose pins or damaged terminals can make a good ECU behave like a bad one.

Module

Internal ECU failure

The ECU itself may have failed internally, especially if voltage and wiring checks all pass.

Safe to drive?

Can you drive with P0605?

Driving with P0605 depends on how the vehicle behaves. If the car starts normally, charges correctly, drives smoothly and only has a stored warning light, a short journey to a garage may be possible.

However, P0605 can become serious quickly. If the engine cuts out, refuses to start, enters limp mode, loses throttle response, shows multiple warning lights or stores voltage-related codes, avoid normal driving until it has been diagnosed.

✅ Lower risk: warning light only, normal starting, no limp mode and stable battery/charging voltage.

⚠️ Medium risk: intermittent warning lights, occasional poor starting, low voltage history or multiple stored codes.

🚫 Higher risk: cutting out, no-start, limp mode, loss of throttle response, burning smell or charging faults.

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

Check voltage before blaming the ECU

One of the biggest mistakes with P0605 is assuming the ECU is finished before checking voltage. A weak battery, alternator fault, poor earth or intermittent ECU power feed can cause memory and processor checks to fail.

If P0605 appears together with P0562 system voltage low, start with the battery, alternator and power supply system before moving to ECU replacement.

If the vehicle has recently had a flat battery, jump start, alternator fault or water leak, mention this to the technician because it can change the diagnostic direction.

Battery Warning Light P0562 Voltage Low Alternator Not Charging
UK repair costs

Typical UK repair costs for P0605

Costs vary because P0605 may be caused by something simple like low voltage or something expensive like ECU repair, replacement and programming.

Diagnostic scan and voltage checks

Typical range: £60–£150.

Battery replacement

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

Alternator testing or charging repair

Typical range: £80–£500+ depending on the fault.

Earth strap or wiring repair

Typical range: £80–£300+.

ECU connector or water damage repair

Typical range: £120–£500+.

Software update or coding

Typical range: £80–£250+ depending on equipment and vehicle.

ECU testing or repair

Typical range: £150–£600+.

Used ECU supply and coding

Typical range: £250–£900+, depending on immobiliser and coding requirements.

New ECU replacement and programming

Typical range: £500–£1,500+ depending on vehicle and module availability.

Repair Costs Guide UK OBD Fault Codes Hub
Diagnosis flow

How to diagnose P0605 properly

A good diagnosis should not start with parts. It should start with evidence. The technician needs to know whether the ECU is genuinely failing or whether the module is being affected by poor power supply, poor grounding, corrosion, software issues or other module communication faults.

✅ Scan all vehicle modules, not only the engine ECU.

✅ Record all stored, pending, historic and communication codes.

✅ Check whether P0605 returns immediately after clearing.

✅ Check battery voltage before starting.

✅ Check voltage drop during cranking.

✅ Check alternator charging voltage at idle and under load.

✅ Inspect main earth straps and engine/body grounds.

✅ Check ECU power feeds with the ignition on and during cranking.

✅ Check fuses, relays and fusebox terminals.

✅ Inspect ECU connectors for corrosion, moisture or loose pins.

✅ Look for water leaks near the scuttle, footwell, fusebox or ECU area.

✅ Check whether any software update or coding history exists.

✅ Check related codes such as P0601, P0602, P0603, P0604, P0606 and P0562.

✅ Confirm ECU failure with specialist testing before replacement.

Real experience

What I would check first

On a real P0605 fault, I would not start by ordering an ECU. I would first check the battery, charging voltage, cranking voltage, earth straps, fuses, relays and ECU connectors.

If the battery has been flat, the car has been jump-started, the alternator has failed, or there has been water inside the car, those clues matter. Control modules are very sensitive to poor voltage and moisture.

Only after the electrical supply and wiring have been proved good would I start thinking seriously about ECU repair, programming or replacement.

P0562 Voltage Low P0606 Processor Fault Diagnostic App
Common mistakes

Mistakes to avoid with P0605

P0605 can be expensive if diagnosed badly. These are the mistakes that commonly waste money.

Replacing the ECU too quickly

A new ECU may not fix the issue if the real fault is voltage, earth, wiring or water ingress.

Ignoring low voltage history

A weak battery or recent flat battery can trigger confusing module faults.

Only scanning the engine module

Other modules may contain voltage, communication or immobiliser clues.

Clearing the code without recording it

Freeze-frame data and related codes can help confirm when and why the fault happened.

Buying a used ECU without checking coding

Used ECUs may need immobiliser matching, key coding and vehicle configuration.

Ignoring water damage

Water in connectors, footwells or fuseboxes can cause repeat ECU and communication faults.

MOT impact

Will P0605 fail an MOT?

P0605 itself is not usually the direct MOT failure item. The MOT concern is what the fault causes. If it causes an engine management light, emissions problem, poor running, cutting out, warning-light fault or unsafe behaviour, it can become an MOT issue.

If the car is in limp mode, running badly or showing a dashboard warning light, it is better to repair and clear the fault before the MOT. If the ECU memory fault keeps returning, the underlying cause needs diagnosing properly.

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

⚠️ Medium risk: intermittent warning light, low-voltage codes or stored communication faults.

✅ Lower risk: fault repaired, no warning light, normal starting and normal driving behaviour.

Engine Light MOT Guide Common MOT Failures
Emissions and readiness

Why ECU memory faults can affect emissions checks

The ECU stores information used for fuel control, emissions readiness and fault monitoring. If a memory fault resets or corrupts stored data, some emissions monitors may not complete properly.

This does not mean every P0605 car will fail an MOT, but it does mean you should not ignore the code if the engine light is on or the car is running poorly.

Engine Management Light Guide Fault Codes Hub
Used car buying advice

P0605 on a car you want to buy

If a used car has P0605 stored, be careful. This is not a code to accept as “just needs clearing”. It may be caused by a simple battery issue, but it can also point to ECU damage, water ingress, software corruption or expensive control-module work.

Before buying, check whether the engine light returns after clearing, whether the car starts from cold, whether the battery and alternator are healthy, whether there are water leaks and whether other ECU or communication codes are stored.

✅ Ask for a full diagnostic report, not just a cleared code.

✅ Check for water damage in footwells, fuseboxes and ECU areas.

✅ Check battery age and charging voltage.

✅ Be cautious if the seller says it only needs a cheap sensor.

✅ Budget carefully if ECU testing or coding is needed.

Used Car Buying Guides Inspection Checklist
Negotiation warning

Do not treat P0605 like a simple sensor fault

Some fault codes point clearly towards a sensor or pipe. P0605 is different because it involves the control module’s internal memory. That can mean deeper electrical, software or ECU work.

If the car is otherwise perfect and the fault is properly diagnosed as a battery or voltage issue, it may still be worth considering. But if the fault is active, repeated, unexplained or linked with no-start symptoms, it is a serious bargaining point or a reason to walk away.

Buying With Advisories Check MOT History
Frequently asked questions

FAQs about P0605

Common questions about P0605 internal control module ROM error, safe driving, repair costs, ECU replacement and MOT impact.

What does code P0605 mean?

P0605 means the ECU, PCM or engine control module has detected an internal read-only memory error.

Is P0605 serious?

It can be serious because it involves the control module. However, voltage, earth, wiring, connector and software issues should be checked before replacing the ECU.

Can I drive with P0605?

Short driving may be possible if the car runs normally, but avoid driving if it cuts out, enters limp mode, refuses to start or shows multiple warning lights.

Does P0605 always mean the ECU is faulty?

No. P0605 can be caused by an ECU fault, but low voltage, water damage, poor earths, wiring faults and programming issues can also trigger it.

Can a weak battery cause P0605?

Yes. A weak battery or unstable voltage can cause ECU self-check errors and memory-related fault codes.

Can an alternator fault cause P0605?

Yes. Overcharging or undercharging can affect control module operation and may trigger ECU memory or processor faults.

Will P0605 fail an MOT?

P0605 can affect an MOT if it causes an engine warning light, poor running, emissions issues, limp mode or unsafe behaviour.

Should I replace the ECU first?

No. Check battery voltage, charging voltage, earths, fuses, relays, connectors, water damage and wiring before ECU replacement.

Can P0605 be cleared?

It may clear temporarily, but if the underlying issue remains, the code will return. Repeated P0605 should be diagnosed properly.

How much does P0605 cost to fix in the UK?

It may cost around £60–£150 for diagnosis, but ECU repair or replacement with programming can rise from several hundred pounds to over £1,000.

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 P0605 guide is designed to help you understand internal control module ROM errors, likely causes, safe driving advice, repair costs, MOT risks and used-car buying concerns before replacing expensive control modules.

Fault codes should always be treated as a diagnostic starting point. P0605 can involve ECU memory, software, battery voltage, alternator output, earth straps, fuses, relays, water ingress, connectors and wiring faults. Proper testing is better than guessing.

For the full fault-code library, always start with the indexed OBD Fault Codes Explained UK hub.