What Is the Difference Between a PCM and ECM?

by Mitchell Brock
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Many people, including professionals, get confused by all the acronyms used in automobile mechanics. A PCM or powertrain control module is different from an ECM or engine or electronic control module, even though most people believe the two are the same. Most modern or newer models only have a PCM, which controls all the engine functions, where the ECM only controlled certain parts of the engine.


Some of older automobile models had two computers, the ECM and the TCM. The ECM controlled the engine function, but the transmission was controlled by a different computer system called the transmission control module, or TCM. The ECM was also used with vehicles that operated with a manual transmission because there was no need for a computer to operate the transmission functions.


The PCM is the main computer in the automobile that controls all the functions of the vehicle. Everything from the vehicle's timing to the proper flow of fuel into the fuel injection system is under the control of the PCM. Along with the engine functions, the PCM controls the functions of the automatic transmission, such as automatic shifting when vehicle is accelerated or downshifting when the vehicle slows down. The PCM does not have as much control of a manual transmission.


Most modern day automobiles, vehicles built after 1996, contain computer systems like the PCM, and it is a very reliable engine component. When the PCM does fail, it can easily exceed $1,000 for replacement, depending on the type of automobile. ECMs do not cost as much because they only found in older models and their function is limited. If a problem is occurring in the automobile, it probably is not the PCM or ECM. A bad battery, fuel filter, bad injectors or burnt ignition coils can be the culprit rather than damage to the PCM or ECM. Once the PCM or ECM is damaged, it must be replaced.

Failure Signs

The automobile will operate differently when the ECM or PCM has failed. Some of the first systems of failure are that the engine will cough, choke or stall, because the PCM is not regulating the fuel mixture going into the fuel injectors. Since the ignition spark and crankshaft position are controlled by the PCM, the vehicle will not start if the computer is not working to control these functions. Almost every warning light on the instrument panel will begin to turn off and on randomly. The computer's monitor systems, such as the ABS and Check Engine warnings, alert the operator. When the PCM or ECM fails, the system is not getting the correct information and this causes all the warning lights to flicker on and off.

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