Definition
An ECU is the electronic control unit that manages a modern diesel engine, reading sensors and controlling injection timing and quantity, boost, protection shutdowns and alarms, and storing fault codes when it detects a problem.
The ECU decides what the engine does moment to moment, within the limits its programming allows. It also protects the engine, which is why some faults produce a power limit — often called limp mode — rather than a failure.
What a fault code is and is not
A fault code names a symptom the ECU observed, not a cause. A code pointing at a sensor reading outside its expected range may mean a failed sensor, a wiring fault, a connector full of salt, or a genuine mechanical problem that the sensor correctly reported. Replacing the part the code names is the most common and most expensive diagnostic mistake.
Reading codes properly generally needs the manufacturer's tooling, which shapes who can work on your boat and where. Gather evidence — conditions, readings, what changed, what the alarm said — and the technician's visit is shorter.
Where you meet it
On any recent engine, in diagnostic conversations, and in the parts-and-support question when choosing an engine for remote cruising.