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Vehicles have an onboard computer called the ECM that acts as the brain of the engine control system. Here are some facts about it.

What Does ECM Stand For?

ECM stands for “electronic control module.” The ECM serves as the engine’s computer and can go by many names, including electronic control unit (ECU), electronic control assembly (ECA), or controller. Ford technicians tended to refer to the Ford engine controller as a “processor” but “ECM” was the GM preferred abbreviation in the early days.

But according to the Society of Automotive Engineers (SAE) bulletin J1930, powertrain control module (PCM) is the standardized term for the onboard automotive computer. The reason for this re-designation rests in the fact that when the J1930 regulation was written, most vehicles had their engines and their electronic transmissions controlled by the same unit. But on any vehicle where the transmission is controlled by a “TCM” or transmission control unit, the abbreviation of the engine controller reverts to “ECM,” since the module only controls the engine.

A car’s PCM has a network of electronic sensors, actuators, and computer modules that regulate the powertrain and other support systems. Like any other computer, the ECM/PCM has inputs that determine its outputs. There are also feedback sensors, like the O2 sensor, that cause the ECM/PCM to modify outputs such as fuel delivery.

Where Is the ECM Located?

Most ECMs can be found in the passenger compartment, positioned under the instrument panel or in a side kick panel. Their location protects them from extreme temperatures, vibration, dirt, or any interference other than water that enters the vehicle.

ECM Functions in a Nutshell

The ECM is the “brain” that keeps the engine and transmission working as they should. It processes data, maintains communications, and makes control decisions that will keep the vehicle operating. The components (engine, transmission, or both) won’t work if this unit is powered down, loses all its grounds, is damaged, or disconnected.

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Other than that, the engine computer also undergoes self-tests, sets, and stores diagnostic trouble codes (DTCs), and communicates with other modules on the vehicle network and can be accessed by a technician via a scan tool. The scan tool becomes a module on the network when it’s connected and can be used as a window into the system for the purpose of diagnosis.

How Does the ECM Work?

Like the average computer, the ECM operates when it receives and processes input, stores information, and produces an output.


The ECM receives input from a device like a switch, button, or sensor. These devices transmit information in the form of voltage signals that undergo input conditioning unless they’re digital. Analog signals are typically converted to digital signals by the input conditioning algorithms, but digital signals don’t typically need conditioning.

Input conditioning basically converts analog to digital and/or amplifies voltage signals that are too small for the computer circuitry.

The PCM sources input from sensors that send information like vehicle speed, throttle position, engine RPM, air pressure, oxygen content, and temperature readings, among others.


Once the PCM receives the input, electronic logic circuits process the voltage signals. These logic circuits turn voltage signals into output voltage signals or commands. Whenever possible, inputs are compared for rationality (such as MAF and TP, or RPM and idle speed control).


Automotive computers have storage space that keeps program instructions in electronic memory.

All vehicles equipped with the second generation of on-board diagnostics (OBD-II) have an electrically erasable programmable read-only memory (EEPROM).

Through reflashing, the program in the computer can be reprogrammed with a scan tool and the right software. Vehicle computers can retain data even when the engine is shut off. Typically, during the reprogramming process, about 18 volts is sent to the EEPROM to erase the existing program and then the replacement program is written in.

Aside from an EEPROM, computers also have random-access memory (RAM). RAM can either be volatile or non-volatile.

Volatile RAM is erased whenever the ignition is turned off. Meanwhile, non-volatile RAM retains its memory even when the battery is disconnected.

Think of RAM as the computer’s workbench where data is processed and adaptive learning, such as fuel trim, idle tables, transmission shift quality, etc., is stored. That information is erased and must be relearned if the battery is disconnected. Other parts of the RAM are erased every time the ignition is turned off. Non-volatile RAM would be like a permanent “book” with specific “lookup tables” the ECM’s CPU uses to make its decisions based on prevailing operating conditions.

Think of RAM as the computer’s workbench where data is processed and adaptive learning, such as fuel trim, idle tables, transmission shift quality, etc., is stored. That information is erased and must be relearned if the battery is disconnected.

Richard McCuistian, ASE Certified Master Automobile Technician

The map of how an ECM/PCM makes decisions can be illustrated in a three-dimensional way by using lots of small square “cells” to build a landscape. It’s like a massive deck of cards, any of which the ECM/PCM can access instantaneously depending on dynamic inputs from the driver, the engine, and the environment.

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different program strategy the ecm pcm uses to maintain operation
Every vehicle computer has one or more maps similar to the one you see here, with each square/cube representing a different program strategy the ECM/PCM selects to handle whatever the ECM needs to do with its outputs to maintain vehicle operation. | Image Source: Richard McCuistian.


After processing the input signals, the computer sends voltage signals or commands to other devices like an actuator, which converts electrical energy into mechanical action.

Once this happens, the actuator can perform various functions, such as adjusting engine idle speed, operating fuel injectors, controlling ignition timing, and altering suspension height.

How Many Computers Does A Vehicle Have?

A vehicle can have more than one computer, which is also called a module or controller. Computers can communicate with one another via their input and output functions.

What Causes an Electronic Control Module to Fail?

Electrical problems as a result of damaged wires or a faulty alternator can cause an ECM to fail. Corrupted or outdated software can also affect the computer’s performance.

There are also environmental factors, such as moisture, extreme temperatures, and high pressure, that can produce the same result.

Symptoms of a Faulty Powertrain Control Module

Aside from an illuminated check engine light, there are several telltale signs that point to a damaged or faulty powertrain control module.

Shifting Problems

Erratic or random shifting could mean that something is wrong with the electronic control module. If left unaddressed, a faulty ECM can also cause the vehicle to get stuck in gear, resulting in drivability issues.

Keep in mind, however, that shifting problems can also be caused by other factors, such as having low transmission fluid. To be sure, you can bring your vehicle to the nearest auto repair shop and have a trained professional inspect your daily driver.

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Stuttering or Stalling Engine

The ECM is responsible for controlling the engine’s timing adjustments, so when your vehicle is stuttering or stalling, it could mean that the computer is experiencing some issues.

Failed Emissions Test

The PCM helps determine the appropriate air-fuel ratio for efficient fuel combustion. A problem with the PCM can lead to an overly rich air-fuel mixture and a low combustion rate.

As a result, too much carbon monoxide and other volatile organic compounds (VOCs) will be released into the atmosphere, which can cause your vehicle to fail its emissions test.

car not passing the emissions test
A problem with the PCM can lead to an overly rich air-fuel mixture and a low combustion rate.

Poor Fuel Economy

A faulty electronic control module isn’t the leading cause of poor fuel economy, but there are instances that it can create the same problem.

A damaged or malfunctioning PCM won’t be able to calculate the right air-fuel ratio, causing your vehicle to consume more fuel than necessary.

Car Won’t Start

A damaged engine computer can also prevent your car from starting. However, keep in mind that there might be other causes that can lead to the same issue, such as a shorted wiring harness, improper timing adjustments, and improper air-fuel ratio.

Electronic Control Module Repair

In some cases, skilled mechanics can rectify short circuits or bad connections to get the electronic control module working as it should.

For ECM issues that involve a software bug, flashing the firmware typically resets it to factory specifications.

Repairing a faulty electronic control module isn’t a DIY-friendly task, so you might want to have a trained professional do the job for you instead. Having a skilled mechanic work on your vehicle’s electronic control module ensures that all steps for diagnosis and repair are followed correctly.

Unfortunately, there might be some cases where the computer is broken beyond repair. Under this circumstance, you’ll probably need to buy a new one, which can cost between $128 and $1,155.

About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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John F

Thank you, Richard McCuistian, for another very informative and easy to understand article. I thoroughly enjoy reading your articles with the informative pictures, charts, and diagrams. This article was especially enjoyable to me, due to my own experience with a bad ECM.

A few years ago, my 2004 Chevy Malibu ran very rough, had poor acceleration and stalled intermittently. It had over 400+ Error Codes and three different mechanics tried to fix the car, but they all, eventually through up their hands. So after doing my own research, I suspected a bad ECM. Replacing the old unit with a new one (which had the latest updates), fixed all of the performance problems and Error Codes, and now, the car runs like new. I would encourage people to not overlook the ECM.

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