A faulty fuel injector can cause a host of problems, including lowered fuel mileage, rough idling, inconsistent power and engine misfires. In extreme cases, it can even allow fuel to leak into the engine, depending on where the failure point is on the component.
Under normal operation, a fuel injector should supply a fine mist of gasoline, just the amount the engine needs to saturate the piston chamber. If the injector is dirty or clogged, not enough fuel will be present, causing a weak firing on that cylinder. It also may leak fuel into the cylinder at the wrong point in the cycle, causing a late or early detonation. If the injector is stuck open, it may spray heavier drops of fuel, reducing fuel efficiency.