Fuel injector installed on the engine. Note connection to the fuel rail on the left and seating into the intake manifold on the right. The electrical connection operates the injector once each engine revolution. If you have a misfiring issue, you can use a stethoscope to check operation of injectors by listening for a slight clicking each time the injector operates. If you have to replace a fuel injector, it’s best to replace them all as a set.