The Fuel Injector is a unit which delivers fuel, under pressure through a nozzle to the cylinder for combustion.

The injectors are fitted close to the inlet manifold, one for each cylinder, the injectors are all calibrated (adjusted) to equally spray a mist of fuel into the intake of each cylinder in a sequence controlled electronically, for the sequential firing of the engine. All diesel engines have fuel injectors, petrol engines may have fuel delivered from a carburetor, where air is mixed with the fuel  in the carburetor before entering the inlet manifold.

Faults occurring with the fuel injector can be blockages, worn nozzles and springs which affect the balance between injectors.