ECU (Engine Control Unit)

The engine control unit (also called ECM or PCM when it controls the transmission too) runs the engine from sensor data: air flow, temperatures, crankshaft and camshaft position, oxygen sensors, pedal position and many more. It decides how much fuel to inject, when to fire each spark plug and how to drive valves, fans and pumps.

It also monitors itself and the emission systems, as required by OBD-II, and stores diagnostic trouble codes with freeze frame data when it finds a fault. Modern cars have dozens of control units talking over a network. Modern cars have dozens of control units, for the transmission, brakes, airbags, body electronics and more. The ECU or PCM for the engine is the one most often meant by the term.

The ECU runs software with calibration maps for each engine. It reads inputs many times per second, calculates outputs and checks every signal for plausibility. The ECU stores maps and calibrations from the manufacturer. Software updates are sometimes released to fix drivability issues or comply with recalls.

Most engine faults are found by reading the ECU’s codes and live data. True ECU failures are rare; codes like P0606 call for checking power supplies and grounds before replacing the unit. ECU failures are less common than sensor or wiring faults, so a diagnosis should rule those out first. A replacement unit often has to be coded or matched to the car and its immobilizer.