Where is the location of the ECU in a car?

In this blog, we will find out the location of the ECU in a car. Also, the answers to the following questions are provided in the blog: what are the basic systems controlled by the ECU? How to replace an ECU?

Where is the location of the ECU in a car?

The ECU or engine control unit is responsible for controlling the internal combustion engine of the car. It is located in the following places:

  • Under the cowl top
  • On the engine 
  • In the fender 

Let’s take a look at each of the locations given below:

Under the cowl top

A cowl top is an area between the engine hood and the windshield of the car. There is space for brake lines and a wiper motor behind the firewall of the engine. The engine control unit is located in this space. It can be on the left side or right side based on the position of the steering wheel in the car.

The engine control unit located under the cowl top requires the removal of components like engine compartment panels, dust filter, and engine control unit bracket. Cooling the engine control unit with coolant lines is not possible as it is located away from the engine.

Despite some disassembly required to access the engine control unit, it is one of the most easily accessible locations of the engine control unit. However, if the water drains on the cowl are blocked then the engine control unit can get damaged due to corrosion of the electrical couplers.

This location is more common in sedans as the main battery is located in the trunk or under the rear seats. Front-wheel drive cars do not have space between the firewall and the windshield so the engine control unit cannot be mounted in this location.

On the engine 

This is the most common location to install the engine control unit in sedans, SUVs, and small hatchback models. The engine wiring harness can be short as the control unit is on top of the engine. Usually, the ECU is installed on the air filter box or in the center of the engine.

The control unit can also be cooled with the help of coolant lines, the ECU housing has a heat sink and metal coolant hoses are routed in such a way that they are in contact with the engine control unit.

ECU cooling is only present in cars with bigger engines, otherwise, the heat sink on the housing is adequate to cool the ECU. in the latest gasoline-powered cars, the output stages for the ignition coils are no longer inside the ECU.

Therefore, the thermal load on the modern ECU is significantly reduced. The ECU only provides power supply to the crankshaft and camshaft position sensor, other power supply is taken from Z-connectors or engine compartment fuse and relay box.

In the fender.

This location is more common in sports utility vehicles, the engine compartment is closely packed as there are more suspension and drivetrain components. The engine wiring harness goes directly inside the front right or left fender.

To remove the engine control unit, the front wheel must be removed along with the fender lining. The fender lining protects the engine control unit from dirt and water coming to form the front wheels.

Due to its difficult location, diagnosis faults if the ECU takes more time compared to the ECUs located in other locations. Special tools like socket boxes or oscilloscopes are difficult to connect unless the control unit is removed completely from the bracket.

What are the basic systems controlled by the engine control unit?

The systems controlled by the engine control are as follows

  • Ignition system

The engine control unit of a gasoline engine is responsible for the ignition system. It consists of ignition coils and spark plugs. The number of spark plugs and ignition coils vary according to the number of cylinders.

The engine control unit determines the ignition angle by calculating the engine load. There are primary and secondary windings in the ignition coil. The actuation signal is sent by the engine control unit to the individual coil according to the firing order.

When the ignition is switched off voltage supply to the primary coils is interrupted and the ignition system is completely off.

  • Fuel system

The engine control unit controls the pressure in the low-pressure as well as the high-pressure fuel system. Fuel pressure is regulated by a pressure regulating valve and quantity control valve.

All the modern internal combustion engines use direct fuel injection, there are electronic fuel injectors connected to high-pressure fuel lines or a common rail. The ignition angle and engine speed determine the start of the fuel injection system.

All the fuel injectors are directly connected to the engine control unit. The ECU monitors the internal resistance of the fuel injectors.

The fuel pressure is measured by a fuel pressure sensor and fault codes are stored along with the fault frame freeze data if the fuel pressure deviates from the specified value.

  • Charge air system

The engine control unit also controls the boost pressure generated by the turbochargers. The boost pressure is regulated by the engine control unit by actuating the wastegate of the turbochargers.

The charged air gets hot as it is compressed by the turbochargers, it is cooled in an intercooler before it reaches the intake manifold. The throttle valve is actuated by the engine control unit according to the position of the accelerator pedal.

In diesel engines, there is an exhaust gas recirculation system. A small number of exhaust gasses is mixed with the charged air to reduce the NOX levels.

How to replace an ECU?

The procedure to replace an engine control unit is very critical as improper handling can permanently damage the engine control unit.

  • Connect the diagnosis scanner to the OBD II port and save the data from the old engine control unit.
  • Turn off the ignition and disconnect the ground terminal of the main battery.
  • Touch a ground point and wear ESD-proof safety shoes before touching the engine control unit.
  • Disconnect the electrical couplers marked M and F from the old control unit and connect them to the new control unit.
  • Connect the main battery and the diagnostic scanner to the OBD II port.
  • Complete the initial start-up of the new control unit, which includes software updates and coding of the new engine control unit.

If the old engine control unit is not communicating due to internal malfunction then the calibration data cannot be transferred to the new ECU. The injector coding and personalization are performed again after the new ECU is installed in the vehicle.


This blog listed the locations where the ECU is installed in a vehicle. The engine control units are manufactured by companies like Bosh, Continental, or siemens. The sensors and actuators present on the engine are connected to the ECU. Hence, the ECU cannot be located far from the engine.