Bad ECU Symptoms: What To Look Out For

Bad ECU Symptoms: What To Look Out For

Whether you call it the engine control module (ECM) or the engine control unit (ECU), this component of the car plays a vital role in keeping it operating. It serves as the primary part for the powertrain and engine, making sure the vehicle runs smoothly. Has your vehicle been acting weird for the past few days? Learn about bad ECU symptoms and how to fix them.

If an engine control unit is faulty, it can give rise to many unwanted symptoms. Today we will talk in-depth about bad ECU symptoms, how to troubleshoot some of those signs, fix them, and replacement costs.

Perhaps the most common bad ECU symptoms are a stalling engine paired with an illuminated check engine light across the dash. You might also see a decrease in fuel consumption amongst other symptoms.

But before that, let’s know a bit more about what an ECU is.

What Is ECU?

An ECU or electronic control unit is a little device fit in a car’s body that can control specific functions of the car. Modern vehicles can contain up to 100 ECUs, more in some cases, and have diverse controlling functions. ECUs can control anything from the basics (power steering and engine) to access and security (keyless entry and door locks), and comfort (HVAC, power seats, and windows).

Active safety features, like automatic emergency braking, as well as passive safety features like airbags, can be added to that long list.

Bad ECU Symptoms

Every ECU generally comprises a dedicated chip that operated its firmware or software and needs data connections and power to run. Different components of the vehicle send in input to the ECU. For example, when a passenger presses the wireless key fob or the door unlock/lock button on a car door, the input would be received by the door lock ECU.

An airbag ECU, on the other hand, gets data from crash sensors. After this, the ECU sends a signal to the actuators to act based on the inputs. In our examples, an actuator would be activated that would unlock or lock the corresponding door. An airbag would be deployed by the airbag ECU.

Types Of ECU

As vehicles come with many ECUs, they are categorized based on the kind of tasks they do. A few of them are:

Engine Control Module

Powered by its sensors, an ECM makes sure there is enough fuel in the engine and the ignition time is just right. This helps a car offer you the most economy and power.

Brake Control Module

The BCM is used in vehicles paired with ABS and its job is to ensure the wheels do not skin and decide when to activate braking and when to release it.

Transmission Control Module

Only found on automatic cars, a TCM gives you the smoothest shifts a car can perform by evaluating the acceleration and engine acceleration of your vehicle.

Telematics Control Module

TCU keeps the onboard services of a car up and running. Moreover, it controls the phone and Internet connectivity as well as the satellite navigation of the car.

Suspension Control Module

Found in vehicles with active suspension systems, suspension control modules (SCM) control the changes to the suspension and proper ride height based on the driving conditions.

Engine Control Unit Location

The ECU can often be found close to or on the engine. In a few cars, you may also locate it behind the glove compartment or under the dashboard. There is no set place for the ECU to be fitted. That information can be found on your vehicle’s service manual.

7 Bad ECU Symptoms

When you notice these signs from your vehicle, do not take them lightly! Troubleshoot the ECU or consult with a professional mechanic immediately.

Bad ECU Symptoms: Check Engine Light

Perhaps the easiest way to recognize something is wrong with your ECU is a “Check Engine” sign illuminated on the dashboard. Certainly, there are many other reasons the warning light would stay on. But in situations where the ECU detects problems with the electronic circuits, components, and/or sensors, it immediately sends a signal to the CEL to illuminate.

Sometimes this mistake is done by the ECU too. To understand if your ECU is truly struggling, you have to look for the particular codes on your car’s computer. You can get to the root of any problem with these codes. Check with your owner’s manual, manufacturer, or various online sources to understand the concept of trouble codes.

Bad ECU Symptoms: Engine Misfires Or Stalls

Another major symptom that quickly follows a Check Engine Light is an engine misfiring or stalling. As there isn’t much consistency with these problems, the pattern of stalls and misfires can be expected to be random, making it harder to narrow it down to a specific problem with the engine.

Many times, this happens as the engine control module fails and starts feeding incorrect information to the engine. If the issue worsens, the engine’s performance will go down significantly.

Bad ECU Symptoms: Poor Gas Mileage

A vehicle needs the proper fuel-air ratio to function optimally. A faulty ECM fails to understand how much fuel must be supplied to the ignition chamber. As a result, too much fuel may be sent to burn. Your vehicle will start running rich and consume way too much fuel.

Bad ECU Symptoms: Decreased Engine Performance

Alternatively, too little fuel can be sent to the combustion chamber if the ECU is faulty. You will notice your car not accelerating like before when you step on the gas. Moreover, it can vibrate and shake, especially when you try driving on an incline, when the vehicle is carrying extra weight, or when something is being towed. You might also notice a rough idle in this case.

Bad ECU Symptoms: Car Won’t Start

The computer of the ECU needs to be functioning properly for the engine to start and operate. Check the other parts of your car if the car doesn’t start. Check the car starter, battery connection, and other necessary components. Only look into the ECU if all other parts are okay.

To check the ECU of your car, you should consult with a professional. If you don’t possess any information or experience handling car computers, we suggest you leave this to the professionals.

Bad ECU Symptoms: Loss Of Acceleration

The ECU has a direct connection to the transmission control module (TCM) as the ECU computer assists the TCM in properly adjusting the car’s acceleration. Thus, you will experience a loss of acceleration if the ECU has gone bad.

The ECU adjusts the throttle using the data gathered by the engine control module (ECM), ensuring the car’s transmission shifts smoothly and effortlessly. So it’s natural that if the ECU is bad, there will be a noticeable reduction in acceleration.

Bad ECU Symptoms: ECU Getting Overheated

The ECU received regular heat from the transistor, processors, or chips. However, the ECU is prone to overheating and that is not good for its health. An overheated ECU cannot handle the car properly and creates issues with the vehicle.

If overheating is a common issue with your engine’s ECU, it will go bad faster. Inspect the heating temperature or level of your car’s ECU.

How To Diagnose Bad ECU Symptoms

Visual inspection and scanner is the least invasive method to check ECM computers. With a scanner, the engine codes can be deduced which will give a rough answer to why your vehicle is being odd.

Bad ECU Symptoms

There are also a few other ways to inspect the ECU of your vehicle, including checking the battery, a basic swap, and the elimination method. You can even use a multimeter to check the ECU.

Following these ways, you can examine the ECM computer. For purposes of keeping this simple to understand, we will discuss three methods in total.

Diagnose Bad ECU Symptoms: OBD Or OBD-II Code Scanner

Every car owner should know what an OBD scanner is, even if they don’t own one. An Onboard Diagnostic (OBD) code scanner can be used to access car data. This tool helps to check the vehicle’s engine and detect all possible glitches in it. Although not mandatory, we highly recommend you buy an OBD scanner.

For this first technique to testing a car computer, you need an OBD or OBD-II scanner tool. Attach the OBD-II scanner tool to your car’s diagnostic part – this will act as the bridge between your vehicle’s problems and you. When an OBD or OBD-II scanner displays codes, you have to identify those codes to repair the problems. Try this video for more info.

These are some of the common codes associated with a faulty ECU.

P0600 Code: According to this code, there is an issue with one of the sensors linked to the ECM. Alternatively, a wiring error can be the culprit.

P0700 Code: If you see this code, know there is a problem with the TCM. When a TCM goes bad, it displays similar symptoms to that of a faulty ECM.

P0606 Code: This code is stored in your vehicle when there is an issue with your car’s PCM. A lot of the signs are similar to an ECM’s but they are only somewhat connected.

Diagnose Bad ECU Symptoms: Visual Inspection

You have to perform a visual inspection on the ECM computer to test this part. The first thing that you will see when you begin a visual inspection is PINs. If you see the PINs are intact and good, you can move on to the second step, which is performing a little smell test on your ECU. Understand something is wrong with the ECU if there is a burnt plastic smell coming from it.

For further visual inspection, however, you may have to dismantle the ECU. Try to remain careful during this step and don’t hurry. After you take apart the car computer board, you have to look for any broken or corroded components in it. If there are any, you have to get them replaced.

Diagnose Bad ECU Symptoms: Car Battery Check

You can rely on your battery voltage to tell you if your ECU computer is functional or not. The sensors linked to the ECU cannot perform the right adjustments if the battery voltage isn’t just right. The battery of a car has to be fully charged when you take it out for a spin.

Use a multimeter to check the car battery voltage. Do it in two conditions – once with the engine on and once with the engine off. If the reading is 13.7 volts when the engine is on and 12.6 volts when the engine is off, your battery is charged fully and in good shape.

If the reading is under 12.6 volts when the engine is off and under or over 13.7 volts when the engine is operating, the battery has to be recharged. Make sure to test it again with a multimeter after that.

Replacement Costs Of Bad ECU

Once you are sure that your vehicle is displaying bad ECU symptoms, it’s time to look into repair or replacement options. For the latter, the cost consists of three factors: checking, reprogramming, and entire replacement. As the ECU is such a complex component, diagnosing it isn’t easy. You have to visit an auto shop to run tests for numerous systems.

Just the tests can be priced in the range of $150 to $300. The price may or may not include labor.

Once the mechanic finds the issues and registers that they can fix them, they will look into the issue. So, after the diagnosis fee, you have to pay between $300 to $750 for reprogramming charges. Modern vehicles come with very specific components which can easily add to the total price.

Lastly, they will issue a full replacement if the tests say that the parts cannot be repaired. This will be a bit cheaper than a reprogram, but the range is pretty close. You will have to pay between $500 to $600. This does not include the price of labor and diagnostic.

Bad ECU Symptoms

What Factors Go Into The Replacement Cost

A lot of shops include the diagnostic charge into the final price, but it can be different from one shop to another. An ECU that controls the transmission and engine is known as a powertrain control module, and it might come with a warranty protecting it. Have a word with your warranty company or the dealership from which you bought the car.

You can save a little amount by replacing the ECU yourself, but the majority of the cost is taken from the parts themselves anyway.

In some cases, the ECU can be reprogrammed or reconfigured. You are lucky if that is the case with you; then you don’t have to buy a new ECU but only incur the labor expenses. For those who own an OBD-II scanner, you can do the diagnosis by yourself.

Economy cars are cheap to repair. Cars featuring complex electronic systems are going to cost more to repair. While you might be tempted by the price tag of a used ECU, it’s not a good option in the long run. Not only can it break down earlier than expected, but it has to be reprogrammed too.

How To Repair A Faulty ECU

Fixing or repairing a bad engine control unit can be daunting if you do not possess adequate knowledge about the component or do not have prior experience handling car parts. And fixing a bad ECU is anything but easy. Without the proper tools and technical know-how, you can very easily mess up the interior of your vehicle.

Warning: Do not try to fix the ECU of your car if you’re a newbie to car repair. Leave the task to a professional if you want the job done correctly.

Here are a few methods to try when attempting to repair a bad ECU:

Ignition Coils

The ignition coils of a vehicle transform battery voltage across the entire car’s electric system from the ignition system of the car. If the ECU in your car is bad, you must inspect the ignition coil to repair or replace it.

Check For Bad Spark Plugs

Bad spark plugs may need to be repaired or changed when the ECU in your car is malfunctioning.

Test Fuel Injection

In case your vehicle is showing bad ECU symptoms, test the fuel injection properly.

Examine Mass Air Flow

Check out your vehicle’s mass airflow sensor. Make sure this component is working properly.

Fix Throttle Position Sensor (TPS)

A vehicle’s TPS, or throttle position sensor, is directly linked to the engine control unit. Thus, if there is anything wrong with the ECU, it also affects the throttle position sensor.

Idle Air Control Valves

Also termed IAC, the idle air control valves have to be looked into when the ECU isn’t working. Poor IACs may be preventing the component from functioning.

Test Grounds

Use a multimeter to test the ground – this is the best way.

As mentioned earlier, fixing a bad ECU isn’t simple. No matter how long you’ve owned a car, you need to have a certain level of experience and knowledge to repair a faulty ECU. This is why we always suggest going to a mechanic when in doubt. Why risk pricier repairs in the future?

Is It Safe To Drive With A Bad ECU?

Driving with a bad ECU isn’t recommended, but you can keep doing it – in theory, that is. The longer you drive the car with a bad ECU, the more problems you are going to have to deal with when you bring the car to an auto shop. Based on the symptoms stated above, if your engine turns off or stalls while moving or at a standstill, you can be in a dangerous situation.

Fortunately, there are plenty of ways to keep an ECU healthy. Regularly maintaining a vehicle goes a long way in keeping all components of the vehicle running smoothly. It includes:

  • Oil changes
  • New wheel bearings
  • New brake pads
  • Having the tank filled at least halfway at all times
  • Fluid top-offs
  • Not pushing the vehicle too much
  • Having adjustments done by a professional

In general, an ECU has a life expectancy of 100,000 miles. But when you properly maintain your car and keep track of it, it allows you to surpass those expectations and move it nearer to the service life of the car. It will also be a great help when you try to resell or trade that car.

How Many ECUs Can There Be In One Car?

We have seen that the number of ECUs in modern cars is on the rise. Companies are more dependent on computers to power a car. We stated before that the powertrain control module controls numerous parts of the car while keeping the count under 10. However, there’s no need to be surprised if there are 30 to 50 units in your 2020 model.

Hybrid or fully electric vehicles rely strongly on these modules. A car could have up to 80 units working in cohesion to make it run. Modules depend on the model and make of the vehicle, but, these are the more common ones:

  • Brake control
  • Transmission control
  • Suspension control
  • Telematics control
  • Engine control

It is normal for ECUs to control power systems like the windows, automatic trunks, parallel parking, seats, automatic braking, as well as blind-spot warnings. These safety features are actual lifesavers when you go driving; just know that all of these come at a price and you might have to learn that the hard way when it’s time to repair them.

Do Old Cars Contain ECUs?

Engine control units started being produced in the early 80s and very few in the late 70s. Bosch was the one that introduced ECUs to the market for the first time as small computer units fitted inside specific cars. By the 80s, General Motors had started incorporating ECUs into their vehicles.

There are advantages and disadvantages to vehicles with ECUs. Older cars would be harder to shop for as they aren’t the manufacturer’s primary selling focuses anymore. Newer cars will have to be brought to the dealership where even the smallest repairs can be pricey.

Facts about Engine Control Unit (ECU) Malfunction:

  1. The ECU, also known as the Engine Control Module (ECM), is the main computer in your car that regulates engine performance.
  2. It receives information from engine sensors to adjust the air/fuel mixture, fuel usage, and combustion sparks.
  3. A faulty ECU can cause engine performance issues, such as engine lags or surges, misfires, and stalls.
  4. Engine misfires and stalls can occur when the order in which the spark plugs fire is compromised by a faulty ECU.
  5. If the ECM has completely died, the car won’t start.
  6. The check engine light is controlled by the ECU and will turn on if the ECU malfunctions.
  7. False check engine light warnings are often attributed to an ECU or electrical problem.
  8. It’s important to get the engine control module tested at the first sign of trouble.
  9. A faulty ECU can cause reduced fuel economy and engine timing issues.
  10. A new car’s ECU controls more than just the air/fuel mixture, fuel usage, and combustion sparks.


If not the lifespan of a car, an ECU should last at least 100,000 miles – given you maintain it properly. Always keep an eye on bad ECU symptoms to identify a faulty ECU. The electric control unit has a steady grip over the performance of an engine. It is time-consuming and pricey to have work done on the ECU, so keep up on fluids, brakes, and your driving technique to remain on the road longer.

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