An engine control unit is the computer that runs an internal-combustion engine. It reads sensors, including air, temperature, and crank position, and it commands fuel delivery and spark. A modern vehicle may have several such computers. This one is the computer for the engine, not the computer for the brakes or the body electronics.
What the Computer Decides
- Fuel. How much to inject, and when.
- Spark. When to fire the plug, given speed and load.
- Limits. It can cut fuel or retard spark to protect the engine.
The Engine Computer and a Sensor
| Compared | The Engine Computer | A Sensor |
| Role | Decides and commands | Reports one measurement |
| Fuel and spark | Commands both | Does not command them |
| A failed reading | Uses a fallback or a fault | Is the failed reading |