What Causes ECU Failure?

Wondering what causes an ECU to fail? Here's the ultimate guide with all the possible reasons and causes on ECU failure.

An ECU (Electronic Control Unit) is no longer a single electronic control box.

Nowadays, cars contain tens or hundreds of separate modules, all feeding back to one central system. These, collectively, are referred to as a vehicle’s ECU.

Modern cars are more and more dependent on well-designed, highly efficient ECUs and electronics. And with good reason – you get more power, lower emissions, better fuel economy, and fewer problems.

However, when something goes wrong, it can be a more serious issue. Most people don’t have the expertise to fix a failing ECU, and an automotive electrician is usually required.

This post will talk about why ECUs fail. The main message is: don’t take chances with your ECU or its wiring. Doing so can create issues.

Related:Symptoms Of A Bad ECU And Replacement Cost

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What Happens When An ECU Fails?


A car’s ECU doesn’t usually outright stop working. It’s much more likely to reduce its efficiency or start playing up in strange ways.

For example, your fuel economy might drop. You might also find that the alarm goes off at odd times or the lights flicker at random intervals.

Check out more information about the symptoms of a bad ECU here (INSERT LINK HERE).

ECU failure is an electrical problem. It’s most often caused by:

  • Moisture
  • Electrical overloads
  • Overheating
  • Poor electronic design

In most cases, the circuit board either breaks or short circuits. As a result, the electrical supply is interfered with, and the ECU stops working as it should.

Note: in extreme cases, a failed, overheating ECU might even start an electrical fire. If this happens, you have far more to worry about than some odd electronics! Don’t attempt to put the fire out, as it can result in serious harm. Wait for the fire brigade.

ECU Failure Causes: Moisture And Water Ingression

Moisture or Water

Water and water vapor in general harms electronics.

When a control module box is designed, it’s created to keep all moisture out. It’s sealed to the outside world, to all intents and purposes.

Of course, where there’s a will, there’s a way. Water could always find a way in. This could be down to the following:

  • Tampering with the ECU
  • Attempted modifications
  • Physical damage (from a crash or nearby component failure)
  • Manufacturer design flaw
  • Supplier production defect
  • Non-OEM parts

Any water making its way into the control box is most likely a form of rainwater or condensation. You could also have problems from other non-natural sources, like car washes.

Water can either hinder the efficient functioning of the circuit or result in the formation of copper dendrites. These connections extend across various parts of the PCB, causing a brief circuit. They subsequently merge and explode, generating a minuscule spark.

Common sense should tell anyone that water and electricity don’t mix. The end result is almost always bad.

ECU Failure Causes: Electrical Overloads

Electrical Overload

Regulating the current in modern cars is a tricky task all by itself. For example, manufacturers dedicate entire teams to creating systems for charging current management.

Every electrical system experiences variations and brief periods of increased (and decreased) electric currents. These are commonly referred to as electrical transients.

An ECU module – and all other electronic components – should have adequate protection against these electrical transients. If they don’t, too much current passes through them, doing irreparable damage.

As well as the transients from the car’s electrical system, there can be other causes – for instance, plugged-in devices or off-board chargers.

There isn’t much additional anyone can do to shield themselves against vehicular electrical transients. However, do ensure that you use all external devices securely. Follow the manufacturer’s instructions to the letter.

ECU Failure Causes: Overheating

Overheating or Hot Temperature

When any electronic component gets too hot, it stops working. The efficiency reduces. In fact, the current could even be shut off to prevent electrical fires from starting.

You’ll most likely have seen this in other everyday applications. For example, in the sun, your smartphone either runs through its battery much quicker or shuts itself off.

If an ECU overheats, it’s usually caused by a design flaw. There could also be physical damage, poor connections, aftermarket modification attempts, and so on.

Overheating ECUs could lead to further problems, such as solder joint failure.

The most common symptom is poorer performance and a reduced overall lifespan.

ECU Failure Causes: Poor Electronic Design

Electrical Design

Poor circuit board design or circuit attached to the module could mean failure.

This poor design could be from the manufacturer – but it could be from an aftermarket modification. The latter is more likely if you don’t see any recalls or open investigations for your vehicle.

There are hundreds – if not thousands – of ways a system could be inefficiently or incorrectly put together. These include (but aren’t limited to):

  • Poor protection against things mentioned above (moisture, electrical overloads, overheating)
  • Defective or damaged electronic components
  • Poor solder joints
  • Using a low-voltage-rated component in a high-voltage circuit
  • Bad connections, grounds, terminals, wiring, etc.

What Else Causes ECU Failure?

Body Control Module ECU ECM BCM

There are so many ways an ECU can fail due to their respective causes. Most of them fit (more or less) into one of the categories mentioned in this article.

Other things that can influence ECU failure include:

  • Circuit board track materials and track spacing
  • The type of solder (and its application process) and flux
  • Circuit board cleaning or no-clean process
  • Where the module is located in the car
  • How the control box is orientated
  • Circuit node impedance
  • and many, many more.

If My ECU Is Bad, What Caused It?

ECU Inside Car

Identifying the root cause of an ECU malfunction is difficult without taking the affected module apart.

When your car goes in for repair, a specialist mechanic will check for evidence of any of the above. They might alter some design aspects when they fit a new unit. However, they could simply tell you that you’ll need a new box every X years.

If you notice any symptoms of a bad control module, you should check for any listed service bulletins. In some instances, you’ll find that the manufacturer has accepted responsibility for a production defect. They’ll fix it for free.

Much of the time, though, there won’t be any official recalls. It’ll be down to you to pay someone to find the problem.

Take your car to an automotive electrician for an in-depth diagnosis and repair. This might involve a new module and could cost up to $1,000, including labor.

Finally, one last reminder. Never mess with your ECU or any control modules. You’ll almost certainly do more harm than good.

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!