What Is An ECM/ECU And Why Is It Important?

ECM thumbIn a modern vehicle, the Engine Control Unit (ECU) is a small computer that controls the way the engine works. It maximizes the efficiency and performance of a vehicle by adjusting the fuel-air mixture and ignition timing.

This guide will explain why a modern ECU is so important, look into its history, and describe how it functions.

What Does An Engine Control Module (or Electronic Control Unit) Do?

ECM ECUImage Credit: ServiceTutorials


You may have heard of the term Engine Control Module (ECM) or Engine Control Unit (ECU) before. Both acronyms refer to the same thing, which is a small computer that controls how the engine operates.

Modern engines have a lot of advanced mechanical and electronic parts designed to improve power and efficiency. The ECU coordinates everything to allow the engine to run smoothly, and produce the maximum amount of power with the most fuel efficiency possible.

Every internal combustion engine needs fuel and air to work. Manufacturers program their ECUs to allow the engine to make the most of this mixture. Many ECUs also communicate with the transmission control unit (TCU). The two control units work together to optimize the transmission operation for maximum fuel economy and drivability.

Factors Influencing ECU Programming


  • Market positioning – The ECU in a high performance car will be programed to maximize performance, perhaps at the cost of less fuel efficiency or less smooth shifting.
  • Emissions regulations – All ECU’s are programmed to make sure the engine meets minimum EPA requirements.
  • Drivability – The ECU works with the TCU to make sure the vehicle performs correctly for it’s customer base.
  • Efficiency requirements – All ECU’s with factory programming are set up to make sure the vehicle gets fuel economy that meets government requirements.

In summary, an ECU optimizes the fuel-air mixture to get the engine to perform as efficiently as possible.

How An ECU Optimizes Your Engine’s Fuel-Air Mixture

  • Adjusting Valve Timing
  • Managing Exhaust Gas Recirculation (EGR)
  • Controlling Variable Intake Manifolds
  • Managing Fuel Injector Timing
  • Regulating Turbo Boost Pressure (when a turbocharger is fitted)
  • Controlling Ignition Timing

Not all these parameters may apply to your vehicle, as each engine and ECU is a unique design.

ECU Development Over The Years


ECUs in cars have been around since the late 1970s. Auto manufacturers had been experimenting with computer-controlled engine management systems for years. But increasing air pollution in cities and the 1973 oil crisis accelerated their efforts to reduce vehicle emissions.

Toyota, BMW, and General Motors were among the first to use ECUs in their vehicles. At first, they controlled basic ignition timings.

By the early ‘80s, microprocessor technology allowed ECUs to take on a far more complex role.

Fuel injection started replacing carburetors, allowing for more precise fuel control. Variable length intake manifolds and variable valve timing also became more common. All were integrated with the ECU to ensure proper operation.

Direct injection, turbocharging, and cylinder deactivation arrived in the late ‘90s and early 2000s. By this time, the ECU had evolved into a far more complex device. It coordinated dozens of mechanical and electrical component – all to ensure that the engine operated as efficiently as possible.

In 1991 Onboard Diagnostics (OBD) was also added into the mix. This system monitors and controls hundreds of engine functions. The ECU compares the outputs to predetermined targets and adjusts settings thousands of times a second.

Modern internal combustion engines couldn’t function without an ECU. The way your car accelerates, shifts, and performs all depends on the programming of the ECU. Without it, we would have worse fuel economy, less power, poor responsiveness, and far higher emissions.

How Does An ECU Work?

A modern ECU looks like a small rectangular box. There is an electronic circuit board inside it that looks much like the internals of your computer.

It can be located just about anywhere a manufacturer wants to put it. Most are in the engine bay or in the front footwell. Some cars have them in the trunk.

The ECU receives information from sensors in the engine and other vehicle control units. It compares these inputs to predetermined parameters. If the readings are out, it will make adjustments as you drive. If the readings are way out, it will log a fault that can be checked with an OBD tool.

If the ECU is faulty, your car won’t work properly. If the ECU fails, your car won’t work at all.

OEM Electronic Control Units From Tom’s Foreign Auto Parts


TOM’S Foreign Auto Parts stocks quality used OEM parts. We have been providing excellent used OEM parts for over 30-years and offer an industry leading guarantee. Saving you money over new OEM parts and giving you peace of mind.

In fact, you can save up to 75% off OEM prices on thousands of auto parts when you shop online with us.

Our Most Popular ECU Units

Toms Foreign Auto parts carries a huge selection of ECMs. Among our most popular makes are Infiniti and Honda – find them here:

We have almost 6,000 OEM quality used ECUs in stock. Contact us online or browse parts by make here if you need help finding the right ECU for your vehicle.