Some of you may remember the days of a choke, clutch, and even manual fuel injection. These systems may be mostly relegated to the past, but their functions are not. Instead, they are computer controlled by an ECU.

The ECU is one of the most vital components in the modern vehicle. It controls ignition timing, fuel injection, and many other essential engine functions. Modern vehicles rely so much on it, they can not function without it.

When the ECU is having issues, you might get a P0605 check engine light code. As with any other trouble code, you need to know how to identify it and what is causing it. This comprehensive guide will show you how to do that and more.

What is the P0605 Code, and What Does It Mean? 

The P0605 code refers to an “ECU ROM Error.” Your ECU has set parameters that each sensor and vehicle functions need to operate within. It stores these in the ROM (read-only memory) chip. Without a functioning ROM, your car will experience all kinds of issues.

What Are ROM Errors?

A ROM error is a storage error. Parameters such as ignition timing, fuel injection, and vehicle operating temperature will be stored on this chip. 

These sensor readings come in as voltage and if they are not within a certain voltage or spike and dip too fast, the ECU logs an error code. Of course, if the set parameters no longer exist, the ECU can no longer tell these sensors what to do and the functions which they control will no longer happen. Almost all modern vehicle systems rely on this system.

What Causes the P0605 Code?

The P0605 code, being an ECU fault, means that all the causes will be related to the ECU. Although this helps us track down the root cause quicker, fixing and diagnosing elevate by degrees. Here are the common causes:

• Battery failure
• Failed ROM chip
• Dead ECU
• Damaged wiring circuit
• Aftermarket ECU upgrade

Battery Failure

Your vehicle contains many fail-safes to prevent damage to the ECU if the battery fails. Car batteries are not lifetime components and will fail eventually, so manufacturers and owners must plan for such a scenario.

However, an incorrect battery, voltage, or overcurrent can cause the P0605 trouble code. An incorrect battery is more common than the other causes as certain vehicles have software locks for aftermarket batteries, preventing you from using any other battery other than the manufacturer.

Failed ROM Chip

Failure of the ROM chip without the rest of the ECU failing is not the most common occurrence. If there is no other damage to the ECU or vehicle and only the chip has failed, it’s likely a factory fault and may be subject to a recall.

Dead ECU

This is fairly common in flood-damaged vehicles or vehicles that have suffered accident damage. Although most ECUs are protected, water damage will eventually corrode the ROM chip and cause this trouble code. You may have thought you struck gold buying a fixer-upper and then this trouble code appears. This is because flood-damaged ECUs may function for a time and then fail.

Damaged Wiring Circuit

Like any other electrical component, the ECU must receive current from the battery and voltage readings from the various sensors. If your wiring harness is damaged, it may not be receiving either of these. With this trouble code, often this occurs at the component level, where the soldered circuit is broken within the ECU.

Aftermarket ECU Upgrade

Aftermarket ECU chips that offer more power for a fraction of the price are common on sites like eBay and Alibaba. However, because they rarely have their own circuits and pull voltage from the ECU, they can cause the ROM chip to malfunction and the P0605 trouble code.

They might also perform changes to the default firmware which will not damage the chip, but cause the ECU to log the ROM error.

What Are the Common Symptoms of P0605 Fault Code?

There are a few common symptoms of the P0605 trouble code. None are too prominent, but you must look at your lights, as they are the key to identifying this code. The most common symptoms are:

• Check engine light
• Limp mode
• Transmission issues
• Failure to start

Check Engine Light

If you have this trouble code, all the lights will probably malfunction, your check engine light included. This is because the light parameters themselves are stored in the ROM chip. DIY mechanics call this the dashboard Christmas tree.

Limp Mode

If you can start your vehicle, you will be sent into limp mode as soon as you get going. The ECU controls many core vehicle functions, so failure can be a major safety hazard. Limp mode prevents you from operating the vehicle at high speed and in certain vehicles, you may not operate it at all.

Transmission Issues

In vehicles that use an automatic transmission, The ECU controls shifting, clutches, and other internal functions. In many vehicles, the transmission module is separate from the ECU, but in some modern vehicles, top-level transmission commands are issued by the ECU. This trouble code could prevent your transmission from changing gears correctly or at all. This is a potential safety hazard, as the vehicle may move forward on its own.

Failure To Start

Ignition timing and fuel injection are both controlled by the ECU. The P0605 code means that neither of these functions can occur, so you may find it difficult to start the vehicle. If you can start it, it may idle roughly and stall.

How to Diagnose and Fix the Cause of P0605 Code 

Diagnosing a failed ECU requires a certain level of understanding of how this unit works. If you’re not sure that you can inspect an ECU without damaging it, it’s best to leave this part of the job to the professionals. That said, you can still run a full diagnostic scan and potentially pinpoint the issue that way.

Does P0605 Clear Itself?

The P0605 code will clear itself once the ECU is fixed, as the main functions of the ECU will be restored. As always, it is better to clear the codes using an OBD-II reader, as this will ensure the issue is fixed.

Is It OK to Drive with P0605?

The ECU controls vital vehicle functions, hence there is no way to drive with this trouble code. Additionally, it is a potential safety hazard as your vehicle may function intermittently and become uncontrollable.

Get the Right Parts for Your Car!

Knowing how to fix and diagnose a P0605 is only half the battle. In reality, you need the physical tools and components to get your vehicle back on the road.

Here at eEuroparts.com, we have all those tools, components, and more for European vehicles. Our easy-to-use interface allows you to select your vehicle and parts that fit your specific vehicle. We are just as eager to get your vehicle back to full health as you are.