What is an ECU, and what does it actually do?

An ECU - Electronic Control Unit - is a small computer that reads sensors, makes decisions and drives actuators. In an engine application it decides how much fuel to inject, when to fire the spark, how much boost to run and when to protect the engine by limiting power.

Because it sits between every sensor and every actuator, a fault inside the ECU rarely announces itself as an ECU fault. It shows up as a symptom somewhere else: a no-start, a sensor code that does not go away after the sensor is replaced, or a module that has simply stopped answering the scan tool.

Common failure modes

Most control module failures fall into a small number of categories.

• Power supply faults - regulators and filtering that no longer hold a stable rail
• Output stage failures - injector, coil or valve drivers that have shorted
• Communication faults - CAN or K-line transceivers that stop the module talking
• Moisture and corrosion - typically where the connector or seal has failed
• Physical and heat damage - cracked joints, burnt tracks, impact damage

Why this matters before you replace it

A replacement module usually has to be programmed or adapted to the vehicle, and on many applications it also has to be matched to the immobiliser. That makes replacement the most expensive path. Confirming the module is genuinely at fault first is almost always worth the time.