An engine control unit (ECU) is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called lookup tables), and adjusting the engine actuators accordingly. Before ECUs, air-fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means.
However, the engine control module might betray a few hints, even in name alone. Sometimes, the names of automotive components are so bizarre that we forget that some terms are completely intuitive and logical. "Engine control" is a no-brainer; "module" implies it's electrical in nature. And if that's not enough to enlighten you, well, at least you weren't the first to ask those questions. If you type "electronic control module," or its better-known acronym, ECM, into the search field, you'll be gently guided over to the ECU (engine control unit) page. And from there it can get confusing, because there's a whole rat's nest of electrical terminology to trace and pick through. Sometimes, the engine control module is at fault for issues that would often be assumed to be "mechanical," like engine noise or problems with the engine running smoothly.