4 min read

What is an ECU (Electronic Control Unit) in a car?

An ECU is a computer that figures out what a moving part, called an actuator, should do by looking at signals sent by different devices.
What is an ECU (Electronic Control Unit) in a car?
Photo by Umberto / Unsplash

Fun fact. A modern car can have up to 150 ECUs. (Research from 2019)

In this article, we will describe an ECU and what it stands for. We'll look into examples of types of ECUs in a car. We'll also illustrate what data flows through it and how it works on a high level. Finally, we'll review some of the requirements the ECU needs to meet and what the future might hold.

Let's start with the Wikipedia description:

An electronic control unit (ECU), also known as an electronic control module (ECM), is an embedded system in automotive electronics that controls one or more of the electrical systems or subsystems in a car or other motor vehicle.

Let's break it down.

An embedded system is a computer system with a processor, memory and input-output peripheral devices.

Suppose we think of a regular personal computer. It receives input signals from devices such as the keyboard and the mouse. The processor in the computer evaluates the signals and determines the output. It can display a letter on the screen or play music in your headphones. The screen and the headphones are examples of output devices.

For the ECU, the input signals can come from buttons and sensors. The output signals include locking the doors or alerting the driver of an object's proximity.

So, the ECU is basically a computer. In its simplest form, the ECU takes in information, analyses it and works out an outcome.

As with computers, the software is responsible for analysing the input and calculating the output signal. The software is stored in the memory, and the processor runs it.


As mentioned, a modern car has a lot of ECUs. They have various purposes and have different complexity levels.

The engine control module (ECM), also known as the engine control unit (ECU), is the most common. Yes, it has the same acronym as the electronic control unit, so let's use ECM for the engine so we are on the same page.

The ECM can use information such as the current speed and engine temperature to control things like the fuel/air ratio, the idle speed and valve timing.

Some of the bigger ECUs are:

  • Engine control module (ECM)
  • Powertrain control module (PCM)
  • Transmission control module (TCM)
  • Brake control module (BCM)
  • Battery management system (BMS)

These are all quite big modules, but ECUs can also be for smaller modules. For instance, for the A/C control unit, door control unit or the driver's seat.

Now that we have a broad understanding of how the ECU works, let's dig deeper into the information that travels to and from the ECU.

Data flow and examples

Input signals

Inputs are the signals or data received by the system. It can be from a sensor, a button, or a remote key.

Digital Signals

Digital signals are usually simple, representing zero or one, off or on. A button press is a good example. If you want the heater on your seat, you push a button. Pressing that button and setting it to 'on' will send a signal (1) to the relevant ECU (Seat Control Unit), which will send an output signal to turn the heater element on.

Now, you might wonder, "What's the need for an ECU in that scenario? Just connect the button straight to the heater!". True, that will work. But what if you want to ensure someone sits in the seat so you're not wasting energy? Cool, let's add a sensor to detect that. Another attempt to save energy can be to have a timer. The heater can only be on for a certain amount of time. Alright, we need to store that logic somewhere. What if you want different levels of heat?

Starting to see why there are so many ECUs in a modern car? All these different functionalities can quickly increase the complexity.

Analogue Signals

Analogue signals, on the other hand, are more complicated than digital ones. They represent more than the two states that digital signals do. An example is a temperature sensor or voltage.

Suppose we take a battery management system (BMS), for instance. It monitors each individual cell's voltage, temperature and health. Each of these quantities can vary for each cell. So, for the BMS to calculate the amount of power available, the total number of cycles possible, etc., is complicated.


An actuator is a component/hardware responsible for moving and controlling a mechanism/system. It requires an energy source and a control device that converts the energy to a mechanical motion.

Examples of mechanical motions are linear, like a door lock moving in and out, or rotational, like a windshield motor moving a certain degree.

Communication interfaces

Another way for data to come and go from the ECU is through the communication channel (buses). These are mainly used to communicate between ECUs, allowing them to exchange information with each other in an efficient manner. For example, when the driver wants to lock all doors and pushes the button, the driver's door ECU will send out a signal to the appropriate channel and the other door ECUs will pick it up and act accordingly. The great thing is that it only has to send one signal for all the ECUs instead of one per ECU.


Because there are a lot of ECUs in a modern car, and they need to be located all around the car.

They can, therefore, be exposed to different and extreme loads, for example:

  • Temperatures from -40 to 125 C
  • Effects of moisture and humidity
  • Fluids as oil, fuel and brake fluid

Because of this, they all need to meet specific requirements to be able to handle that.

These requirements are related to functionality, quality and service life and can include the following:

  • Reliability in case of voltage fluctuations.
  • Resistance to electromagnetic radiation
  • Service life of 15 years
  • Reach kilometrage of 240000 km

These requirements are standardised under the ISO 16750 standard. Each car manufacturer may also have its own standards in addition to these.

The requirements will continue to grow with increased functionalities and demand for better performance.


With requirements rising, the number of ECUs in a car is expected to increase and be more complex.

Nevertheless, with improved performance capabilities of the electronics components, systems can combine into a single ECU.

This only means one thing (Probably not, just trying to be dramatic).

The demand for skills in this area will likely be in great need so automotive manufacturers and suppliers can continue offering the best solutions.


To sum up. An ECU is a computer that determines an action an actuator should do based on the input signals from different devices.

With multiple ECUs in a modern car, they are constantly talking to each other and sharing information to help with making the best decision.

And with the requirements constantly growing, the demand for building more efficient and high-performance ECUs increases.