This engine speed sensor monitors the rotational speed of the engine crankshaft or camshaft and sends the signal to the engine control unit. It is mounted in the engine block, typically near the flywheel or timing gear, where it reads a toothed reluctor wheel passing in front of its magnetic tip. A failing sensor produces an erratic or missing tachometer reading and may trigger a no-start or derated-power condition. The original sensor is machined to the same air-gap tolerance as the factory assembly, so no shimming or bracket adjustment is needed during replacement.