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The separate driver unit that some engines use to switch the injectors, rather than driving them from the engine module directly. It exists because piezo and high-current solenoid injectors need drive voltages well above the vehicle's supply — the module steps voltage up and delivers a precisely shaped current pulse — and because that work generates heat best kept away from the main processor.

Knowing whether the engine has one changes the diagnosis entirely, because a failed output stage there produces a circuit or range/performance code on the cylinder it drives while the injector and its wiring are perfectly sound. It is the reason an injector fault that survives a new injector should prompt the question of what is actually driving it.

They fail from heat and vibration, usually one output stage at a time, so the pattern is faults on one or two cylinders rather than all of them. Many require programming or coding when replaced, and on some vehicles the injector calibration codes live in this module rather than in the engine module — so they must be transferred, or every cylinder ends up mistrimmed at once.